The aim of this work is to study the safety and the effect of addition of ketoconazole to experimental kidney transplanted rat treated with tacrolimus and predicts the percentage of tacrolimus dose reduction. The material of this work included 60 male Sprague Dawely rats subjected to renal allotransplantation. They were equally divided into five groups: Group I: served as control group, Group II: received FK506 3.2 mg/kg/bw, Group III: received FK506 2 mg/kg/bw, Group IV: received FK506 1 mg/kg/bw, Group V: received FK506 1 mg/kg/bw plus Ketoconazole 20 mg/kg/day. FK506 trough level and laboratory investigations were determined at 0, 3, 7, 10, 14, and 27 days post-transplantation. In all groups loss of body weight was observed at day 27 after treatment compared to that before transplantation. Serum creatinine significantly increased at day 27 compared to the basal level in groups treated with 1.0 and 3.2 mg FK506 (1.80 +/- 0.50 versus 0.39 +/- 0.06 P = 0.001) and (1.03 +/- 0.26 versus 0.50 +/- 0.07 P = 0.001) respectively, while for 2.0 mg or 1.0 mg plus keto groups, no significant differences in serum creatinine levels over time (0.56 +/- 0.22 versus 0.44 +/- 0.10 P = 0.106) and (0.55 +/- 0.30 versus 0.42 +/- 0.08 P=0.160) were observed. Concomitant administration of Ketoconazole and FK506 in transplanted rat model is safe and results in increase of blood trough level concentration of FK506 with 50% reduction of its dose.