The widespread use of pharmaceuticals for the treatment of humans and animals has resulted in the pollution of surface waters with many different drugs. These chemicals may have negative effects on aquatic biota. The current study was undertaken to investigate the effects of two widely prescribed pharmaceuticals, gemfibrozil (GEM) and atorvastatin (ATV) on the behaviour of adult zebrafish and their response to predator alarm chemicals following 7, 14 and 28 days exposure. The effects of the drugs on the metabolic rate and total cortisol concentration of the fish were also measured at the end of experiments, In the first part of experiment, zebrafish were exposed to two different concentrations of GEM (2.5 and 250 ug/L) and ATV(.075 and 7.5 ug/L), alone and in combination. Zebrafish's behaviour was measured before and after adding alarm chemical after 7, 14 and 28 days. There was little effect on the swimming behaviour of the fish before addition of the alarm chemical; however, both chemicals affected their behavioural response to predator alarm. The strongest effect was at day 14 compared to day 7 and 28. Compared with controls, exposure to ATV greatly reduced swimming speed and distance between fish, but increased the distance to arena wall. The lower concentration of GEM had little effect on behaviour; however, the higher concentration slightly reduced the predator-induced response. For the second part, respiration rate was measured pre- and post-alarm chemical after 28 days. Before exposure to the predator alarm, the control fish had the lowest metabolic rate. After the addition of the alarm, the metabolic rate in controls increased while the rate in other treatments decreased. In comparison to the controls, metabolic rate decreased in all groups exposed to GEM, ATV or the combination. Finally, whole body cortisol content of the zebrafish was measured following 28 days exposure to ATV and GEM. GEM slightly increased cortisol relative to the other treatments (P = 0.022). However, there was no effect of ATV or the combination treatment on whole body cortisol. The results show that pharmaceuticals in the environment can cause behavioural changes that detrimental to fish. Also, further studies are needed to investigate the effect these drugs on molecular level that connected to behaviour.