A precise estimate of the postmortem interval (PMI) is one of the primary tasks in forensic pathology. After death, it cannot be assumed that oxidant/antioxidant balance be adequately controlled by the body, so changes in oxidant/ antioxidant parameters are evaluated as biochemical disturbances. Aim: The aim of this work was to investigate whether the changes in some oxidant and antioxidant parameters can be relied upon in estimating the early PMI (the first 8 hours after death) in rat brain and liver. Methodology: Forty rats were divided into eight groups (five rats each). Rats were sacrificed by cervical dislocation after being anaesthetized by ether inhalation. The head and the abdomen were opened, where both brain and liver were extracted at intervals of one hour (0–1, 1–2, 2–3, 3–4, 4–5, 5–6, 6–7 and 7–8 hours intervals). In brain and liver homogenates, the following parameters were measured; the antioxidant parameters: superoxide dismutase (SOD), Glutathione-S-transferase (GST), Glutathione Reduced (GSH) and catalase (CAT), and the oxidant parameters: Malondialdehyde (MDA) and nitric oxide (NO). These parameters were compared between groups depending on time after death. Results: The results indicated that the oxidative damage started earlier in the brain than in the liver and the antioxidants were affected more early in the brain than in the liver to overcome the oxidant damage occurred. There was a statistical significant negative (inverse) correlation between the time after death (P.M. interval) and all analyzed antioxidants (SOD, GST, RGSH and CAT) in the brain, while in the liver, the correlation was between the time after death (P.M. interval) and one of antioxidant parameters (GST). On the other hand, there was a statistical significant positive (direct) correlation between the time after death (P.M. interval) and all analyzed oxidants (MDA and NO) in both brain and liver. Regression analysis was developed in order to determine the equations for P.M. interval estimation using these parameters individually and in combination which selected by the multiple regression analysis of the statistical program. Conclusion: This study concluded that the oxidants and antioxidants in brain and liver of rats are important in determining the early period (the first 8 hours) after death. The liver was found to be more resistible to oxidative damage than the brain. Using these biochemical markers, it is possible to estimate the early postmortem interval.