This study investigated the effect of different photoperiods on acetaminophen-induced hepatotoxicity in rats. Twenty four adult male rats (average weight = 160±7g) were conditioned to different photoperiod regimens for 6 weeks. At the end of the 6-week period, rats exposed to normal, short and long photoperiods received oral acetaminophen (2g/kg body weight) while in the control group, exposed to normal photoperiod, received oral saline. Rats were sacrificed 24 h after acetaminophen administration by cervical dislocation and blood was collected by cardiac puncture for the estimation of liver enzymes activities. Liver tissues were excised and homogenized for estimation of liver malondialdehyde (MDA) concentration. Elevation of serum levels of alanine and aspartate transaminases and alkaline phosphatase caused by acetaminophen intoxication were not affected in rats subjected to long photoperiod while these parameters were significantly (P
