Melatonin has been recently shown by various in-vivo and in-vitro studies to exert potent neutralising effects on hydroxyl radicals, stimulate glutathione peroxidase (GSH-Px) activity, and protect catalase (CAT) from the destructive activity of hydroxyl radicals in neural tissue. We aimed to investigate the possible effects of pharmacological dose of melatonin on some of the antioxidant defence systems in an in-vivo study of experimental spinal injury. Seven groups of adult male Sprague Dawley rats were used in the following scheme: Group I: Naive (n = 6), Group II: Lesion (n = 8), Group III: Melatonin (n = 5), Group IV: Melatonin + Lesion (n = 8), Group V: Placebo + Lesion (n= 5), Group VI: Sham operation (n = 5), and Group VII: Placebo (n = 5). Experimental spinal injury was induced at level T-7-T-8 by 5 sec compression of the total cord with an aneurism clip on anaesthetised and laminectomized animals. The total 10 mg/kg dose of melatonin (Sigma) dissolved in alcohol-water was administered i.p. four times in 2.5 mg/kg doses, at 20 min pre-, at the time of and at 1h and 2h post-compression. At 24+/-2 h postinjury, the rats were euthanized and the lesioned segments of cord were dissected and homogenised with special care taken to distribute equal amount of injured tissue in each sample for analysis of reduced glutathione (GSH), oxidised glutathione (GSSG), super oxide dismutase (SOD), and CAT activity. Compression injury decreased GSH/GSSG ratio significantly (p < .0001). Melatonin, by itself, significantly decreased GSSG content (p < .05) and increased CAT activity(p < .05) in the naive rats. Melatonin treat ment decreased GSSG activity, thus elevating GSH/GSSG ratio, and also increased SOD and CAT activity without reaching statistical significance in the lesioned animals. In conclusion, pharmacological dose of systemically applied melatonin seemed to support some features of the antioxidant defence systems in our hands.