Administration of cyclosporine to rats has been shown to impair testicular function, resulting in a decrease in sperm counts and fertility. In order to determine whether or not the deleterious effects of CsA could be reversed by hormonal therapy, mature male Sprague Dawley rats were treated with CsA (40 mg/kg/day, s.c.) alone or in combination with human chorionic gonadotropin (hCG) (5 μg/day/r; s.c.) for 14 days. Cyclosporine administration decreased the body weight (290±5.30 vs. 339±8.7 g;P<0.05) and reproductive organ weights (testis 1.49±0.42 vs. 1.60±0.03 g; epididymis 0.41± 0.02 vs. 0.49±0.002 g; seminal vesicle 0.61±0.09 vs. 1.60±0.05 g; prostate 0.28±0.04 vs. 0.60±0.06 g;P<0.05) testicular sperm counts (5.80±0.42 vs. 8.49±0.48 ×107/100 mg tissue;P<0.05) and epididymal sperm counts, (28.2±0.95 vs. 51 51.62±2.17 ×107/100 mg tissue;P<0.05) and fertility (25% vs. 100%). Serum levels of LH were elevated (101.98±21.48 vs. 25.6±5.18 ng/ml;P<0.05) and testosterone was decreased (0.48±0.07 vs. 2.06±0.56 ng/ml;P<0.05). The administration of hCG to the CsA-treated rats restored the reproductive organ weights (testis 1.56±0.043 g; seminal vesicle 1.04±0.05 g; prostate 0.70±0.06 g) and sperm counts (testicular 7.88±1.0 ×107/100 mg tissue; epididymal 59.86±4.16 ×l07/100 mg tissue;P<0.05) Serum levels of testosterone (18.63±4.45 ng/ml) and LH (431.65±31.41 ng/ml) were significantly elevated, as compared with control and CsA-treated groups (P<0.05). All the rats in the gonadotropin-treated group were fertile, as compared with 25% in the CsA-treated group.CsA reduced the kidney weight (1.17±0.02 vs. 1.27±0.03 g;P<0.05) and increased the levels of serum creatinine (0.97±0.07 vs. 0.59±0.03 mg/dl;P<0.05): these changes were ameliorated by the administration of hCG (kidney weight 1.35±0.03 g; creatinine 0.76±0.09 mg/dl).