The contraceptive efficacy and reversibility of the chloroform extract of the seeds of Carica papaya in adult male rabbits were investigated. Eighteen adult male rabbits were divided into three groups of six animals each; Group I--control, Group II--administered chloroform extract of the seeds of Carica papaya at 20 mg/animal/d for 150 d by gavage, and Group III--administered the seed extract at 50 mg/animal/d for 150 d. Body weight and organ weight, semen analysis, sperm morphology by scanning electron microscopy, semen biochemistry, histology of the testis, haematology, serum clinical biochemistry, and the fertility status of the control and the treated animals were evaluated. Body weight and the weight of the testis, epididymis, seminal vesicles, and prostate did not show appreciable changes. Sperm concentration showed a gradual decline, reached severe oligospermia (fewer than 20 million/mL) after 75 d treatment, and attained uniform azoospermia after 120 d treatment. Sperm motility and viability were severely affected after 45 d treatment and reached less than 1% after 75 d treatment. The morphology of the spermatozoa by scanning electron microscopy revealed membrane damage in the acrosome, bent midpiece, coiled tail, and detached head and tail. The levels of fructose, glycerylphosphorylcholine, acid phosphatase, and lactate dehydrogenase in the seminal plasma were unaltered. Histology of the testis revealed arrest of spermatogenesis beyond the level of spermatocytes. No toxicity was evident from the haematology and serum biochemistry parameters. The libido of the treated animals was unaffected and the fertility rate was zero. The effects were comparable in both the dose regimens (Groups II and III) and were restored to normal 45 d after withdrawal of the treatment.