Background: Acylation stimulating protein (ASP) is a small protein isolated based on its function as a potent lipogenic factor. ASP, and its recently discovered receptor (C5L2), have been linked to obesity and several metabolic disturbances in humans and mice. Adipose metabolism is mediated through a complex network of hormonal signals including reproductive hormones. ASP was found to be strongly associated with female hormone alterations in vivo suggesting that ASP levels and function may be altered in association with sex hormone changes. Aim: In this study, we aim to investigate the effect of different hormone treatments on ASP levels in in vivo and ex vivo visceral and subcutaneous explant adipose tissue cultures. C5L2 expression will also be measured in subcutaneous and visceral fat depots in response to different hormone treatments. Furthermore, acute effects of different hormone treatments on postprandial AŞP levels after a fat load will be investigated. Methods: Ovariectomized (OVX) rats were divided into 5 groups (n=10) that received different sex hormone treatments for 2 weeks. Fasting plasma ASP levels and lipid profile were measured. ASP levels were measured in the media of subcutaneous and visceral explant tissue cultures to represent the microenvironment of ASP production. The expression of the C5L2 receptor in visceral and subcutaneous adipose tissue was measured by real time PCR, Postprandial ASP levels and lipid clearance were measured in ovariectomized rats divided into 4 groups including controls that were injected with a single dose of different sex hormone treatments. After one hour, a fat load (olive oil) was given, and plasma blood samples were collected at several time points for ASP and TG measurements. Analysis of variance (ANOVA) was performed to detect significant differences before and after and at different time points (within), and between hormonal treatments. Similar analysis was performed for analysing differences between subcutaneous and visceral fat tissue ASP production and receptor expression, Results: OVX rats had a significantly higher mean body weight than the sham counterparts with intact ovaries. Rat weights and TG levels in the OVX rats were shown to be largely influenced by hormone treatments, mainly estrogen which reduced the weights of the OVX rats and increased their TG levels reflecting a lipolytic effect resulting in decreased body weight and adipocyte cell size. The progesterone treated rats maintained their increased weight (due to ovariectomy), that was similar to the control weights and their TG levels also remained unchanged. Plasma ASP levels showed an increased trend with progesterone and P&E treatment and decreased with estrogen which did not reach significance. Interestingly however, plasma TG levels coordinated negatively with this ASP trend, most obvious in the estrogen treated group that showed the highest TG levels corresponding to the lowest ASP levels suggesting delayed TG clearance, while progesterone treatment showed lower TG levels reflecting enhanced TG clearance possibly due to increased ASP levels. In the quest to investigate hormone effect on direct ASP production from fat tissues of the treated rats in tissue cultures, ASP production from the P&E treated rats was the most pronounced (17% increase compared to controls, P<0.02, RM ANOVA). On the other hand, CSL2 receptor expression was markedly higher in the estrogen treated group (7 fold, P=0.034) compared to the controls and the progesterone treated group. In the postprandial study, the progesterone treated group had the highest significant postprandial ASP increase at two hours compared to basal levels and to the controls (439.8+ 62.4 vs 253.45€ 59.03 ug/ml), P=0.04. Interestingly, increased postprandial ASP levels coordinated negatively with corresponding TG levels further suggesting a role for progesterone in enhanced TG clearance. The findings of this study suggest that sex hormones may have a significant role in modulating ASP levels and corresponding TG levels affecting fat storage in vivo. Thereby, increased production of ASP appears to be mostly from subcutaneous adipose tissue influenced mainly by progesterone (in the presence of estrogen). On the other hand, increased ASP receptor levels in response to estrogen treatment may result from increased circulating TG levels which upregulated receptor expression and increased ASP binding, therefore decreasing plasma ASP levels. A reflection of these findings appears in the functional postprandial study, where progesterone enhanced ASP production and TG clearance in a simultaneous manner. The strong association of postprandial ASP levels and TG clearance in the progesterone treated group support the notion of a stimulatory role for progesterone on ASP mediated TG clearance. Conclusion: This is the first functional study to demonstrate a cause-effect relationship between hormone treatment and ASP levels in vivo. The overall findings are promising and may contribute to further understanding the mechanism of female hormones through enhancing ASP production and plasma levels.