Cell cycle analysis using flow cytometer (FCMX) was performed to evaluate the influence of cyclosporin A (CsA) on cellular proliferation during compensatory renal growth (CRG) in uninephrectomized (Ux) rats. CsA (50 mg/kg/day) was administered subcutaneously for 7 days and animals were scarificed 0, 12, 24, 48, 72, 120 and 168 h after Ux. The percent changes in the S (DNA synthesis) and G2M (DNA replication) phases in the contralateral kidney were analyzed. The cortical cells entering the S and G2M phases showed significant differences between CsA and vehicle groups (p = 0.0001, p = 0.001, respectively), but the medullary cells entering the S and G2M phases showed no significant differences between the 2 groups (p = 1.000, respectively). In the cortex, the CsA group showed a significant decrease in the S (at 12, 72 and 120 h) and G2M phases (at 24, 72 and 120 h) compared to the vehicle group (p < 0.05, respectively), but there was no significant difference in the medulla. In conclusion, cellular proliferation is suppressed in cortical cells by CsA.