Short day photoperiod enhances cellular immunity, cortisol production and melatonin secretion, especially in the laboratory animals (Pandi-Perumal et al., 2006; Bilbo et al., 2002). However, field studies of seasonal changes in immunity typically report reduced immune function during the short photoperiod of winter (Sinclair and Lochmiller, 2000). Acetaminophen (Paracetamol, PCM) is widely used as an over-the-counter analgesic and antipyretic drug. Intake of a large dose of PCM may result in severe hepatic necrosis. Therefore, this present study was carried out to investigate the liver protection effect of short photoperiod against acetaminophen-induced hepatotoxicity. Twenty four adult male rats weighing 160±7g were conditioned to different photoperiod regimens for 6 weeks. Group 1, control group, Normal Photoperiod (NP) (12:12, light on from 07:00hr to 19:00hr). Group 2, NP (12:12, light on from 07:00hr to 19:00hr) treated with acetaminophen. Group 3, Short Photoperiod (SP) (8:16, light on from 09:00hr to 17:00hr) treated with acetaminophen. Group 4, Long Photoperiod (LP) (16:8, light on from 05:00hr to 21:00hr) treated with acetaminophen. At the end of the 6th week, rats in groups 2, 3 and 4 were administered oral 2g/kg of PCM solution, while the rats in the control group were administered saline by way of gastric catheter. 24 hours after PCM administration, the rats were sacrificed by cervical dislocation and blood was collected for liver enzymes estimation and liver tissues were collected and homogenised for estimation of lipid peroxidation. The estimated parameters were aspartate aminotransferase (AST), alanine aminotransferase (ALT), alkaline phosphatase (ALP), gamma glutamate transpeptidase (GGTP) and malondialdehyde (MDA) as an index of lipid peroxidation. Administration of PCM caused a significant (P<0.001) elevation of liver enzymes (AST, ALT, ALP and GGTP) level and lipid peroxidation marker, malondialdehyde (MDA) in all the treated groups (groups 1, 2 and 3) when compared to the control (group 1) (See Table 1). However, there was a significant (P<0.01) decrease in the serum levels of the liver enzymes concentration and MDA in the group 4 (SP group) compared to the rats in groups 2 and 3. It was concluded that the short day photoperiod has hepatoprotective effect and this could be due to its inhibitory effect on hepatic tissue lipid peroxidation.