This study describes the subacute and subchronic effects of two plant growth regulators (PGRs) [abcisic acid (ABA) and gibberellic acid (GA(3))] on serum marker enzymes [aspartate aminotransferase (AST), alanine aminotransferase (ALT), creatine phosphokinase (CPK) and lactate dehydrogenase (LDH), (c)-glutamil transpeptidase (GGT)], antioxidant defense systems [reduced glutathione (GSH), glutathione reductase (GR), glutathione peroxidase (GPx), superoxide dismutase (SOD), glutathione-S-transferase (GST) and catalase (CAT)] and lipid peroxidation level (Malondialdehyde = MDA) in various tissues of rats. Rats (Sprague-Dawley albino) were exposed to 75 ppm (parts per million) of ABA and GA(3). Seventy-five parts per million of PGRs as drinking water was administered orally ad libitum for 25 and 50 days continuously. The PGRs treatments caused different effect on the serum marker enzymes, antioxidant defense systems and the content of MDA in comparison to those of control rats. Results show that ABA caused a significant decrease in serum LDH and CPK activity with both periods. Also, GA(3) significantly decreased serum AST, CPK, and LDH activity with subacute and decreased serum ALT, CPK, LDH, and GGT treated with subchronic periods. The lipid peroxidation end product MDA significantly increased in the erythrocyte, liver, brain, and muscle of rats treated with both the period of GA(3) without significantly change in the erythrocyte and muscle of rats treated with the subacute period of ABA. The GSH levels were significantly depleted in the erythrocyte and brain of rats treated with both the period of GA(3) without any change in the erythrocyte, liver, brain, and muscle of rats treated with both the period of ABA. Also GSH levels in the muscle significantly depleted with the subchronic period of GA(3). Antioxidant enzyme activities such as SOD significantly decreased in the erythrocyte, liver and brain tissues but increased in the muscle tissue of rats treated with both the periods of GA(3). Meanwhile, SOD significantly decreased in liver and brain, and increased in muscle of rats treated with both the period of ABA. While CAT significantly decreased in the all tissues of rats treated with both the period of GA(3), decreased in the liver and muscle of rats treated with both the periods of ABA too. On the other hand, the ancillary enzyme GPx and GR activity in the erythrocytes, liver, brain and muscle were either significantly depleted or not changed with two periods of PGRs. The drug metabolizing enzyme GST activity significantly decreased in the brain of rats treated with subacute period of PGRs but increased in the erythrocytes of rats treated with subacute period of GA(3). As a conclusion, ABA and GA(3) had significantly increased the activity of hepatic damage enzymes. Also the rats resisted to oxidative stress via antioxidant mechanism. However, the antioxidant mechanism could not prevent the increases in lipid peroxidation in rat's tissues. These data, along with changes, suggest that PGRs produced substantial systemic organ toxicity in the erythrocyte, liver, brain, and muscle during the period of a 25-day subacute and 50-day subchronic exposure. (c) 2006 Elsevier Inc. All rights reserved.