Sodium nitroprusside (SNP) is an antihypertensive drug with proven dose-dependent toxiceffects attributed mainly to the production of cyanide but also excesive nitric oxide (NO) and derivedreactive species. The present study evaluated whether melatonin administration would have time-dependentprotective effect against SNP−induced toxicity. Male Swiss mice were used in this study.Control mice were treated with 0.9% NaCl; the second group was injected with 10 mg melatonin(MEL)/kg body weight (b.w.); the third group was given SNP at the dose of 3,6 mg/kg b.w.; the fourthgroup received both MEL and SNP at the same doses. In homogenates of brain, liver and kidneys,activities of superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GSH-Px) wereestimated after 3, 6 and 24 h of... drugs administration. The concentration of reduced glutathione(GSH) was also evaluated in the blood, brain, liver and kidneys of mice at the same time intervals.In animals receiving MEL, the highest levels of GSH were observed in all the organs as compared tothe control after 3, 6 h. Meanwhile, SNP decreased GSH concentration in the blood, brain, liver andkidneys in all time intervals. Administration of MEL in combination with SNP increased the GSHlevels in all organs, as compared to the administration of SNP alone; this effect was observed after 3,6 and 24 h. The activity of SOD, CAT and GSH-Px in the MEL-treated group increased after 3 h inall the organs, while in liver and kidney the increase was also observed after 6 h. Conversely, the SNPintoxication caused a decrease of the activity of enzymes in the tested organs in all intervals, whileadministration of MEL + SNP resulted in increased activities of SOD, CAT and GSH-Px in all theorgans after 3 h and 6 h. The investigation carried out in the present study provide new data to addto the study of antioxidant properties of MEL and SNP-induced oxidative stress with regard to time-dependentproperties in different types of tissues.