This study investigated the effects of Nigella Sativa L. (NS) and Urtica Dioica L. (UD) on the number of Mast Cells (MCs) in Carbon tetrachloride (CCl4)-treated rats. A total of 35 healthy male Sprague-Dawley rats were divided into 5 equal groups. All groups, expect control group, were injected subcutaneously with 0.8 mL CCl4 per kg body weight twice a week for 45 days and then CCl4 + UD-treated, CCl4 + NS-treated and CCl4 + UD + NS-treated rats received a daily intraperitoneal injection of 0.2 ml, kg(-1) NS or/and 2 mL kg(-1) UD oils for 45 days starting on day 46. Thymus, spleen and mesenteric lymph nodes were sampled, stained with 1% aqueous toluidine blue and examined microscopically. The thymus, spleen and lymph nodes of CCl4- exposed animals contained significantly increased numbers of MCs compared with control group, CCl4 + UD-treated, CCl4 + NS-treated and CCl4 + UD + NS-treated rats (p<0.05). However, when exposed rats were also treated with UD, NS or UD + NS, a decrease in MCs was observed. These results demonstrate that UD and NS decrease the number of MCs induced by CCl4 in the lymphoid tissues of rats.