The effects of zinc (Zn) and/or melatonin supplementation on cellular immunity were investigated in rats infested with Toxoplasma gondii. Fifty Sprague-Dawley male rats were used for this study. All animals were fed a normal diet, ad libitum, containing 97 mg Zn/kg. They were divided into five experimental groups, as follows. Group I (n = 10) received intraperitoneal injections of zinc sulfate at a dose of 3 mg/kg/d for 3 wk. Group II (n = 10) received intraperitoneal injections of melatonin at a dose of 3 mg/kg/d for 3 wk. Group III (n = 10) received intraperitoneal injections of zinc sulfate (3 mg/kg/d) and melatonin (3 mg/kg/d) for 3 wk. Group IV (n = 10) was infested controls. Group V (n = 10) was healthy controls. There were no differences in the percentage of CD3+ lymphocytes among all groups. For groups I-III, the CD4+ and CD8+ ratios were higher than those of the groups IV and V controls (p < 0.01). Similarly, the total lymphocyte ratios in groups I-III were higher than those of infested and healthy controls (p < 0.01). The total lymphocyte ratios in group III were significantly higher than those of groups I and 11 (p < 0.01). The plasma Zn levels in the supplemented groups were significantly higher than those of control groups IV and V (p < 0.01). These results suggest that melatonin and/or Zn supplementation may activate cellular immunity by stimulating CD4+ and CD8+ production in infected rats with T. gondii.