In recent years, 6-MP treatment has been beneficial in the treatment of inflammatory bowel disease (IBD). Since 6-MP and its metabolites interfere with various steps in nucleic acid biosynthesis, chronic use of 6-MP could theoretically alter normal cell turnover, including spermatogenesis. Therefore, we have investigated the effect of daily 6-MP administration on spermatogenesis in the young rat. 6-Mercaptopurine was administered in clinically relevant doses 24 and 40 mg/m2. Testicular weights of rats treated with 24 mg/m2 for 75 days or 40 mg/m2 for 25 days were not significantly different among 6-MP, pair-fed, or ad libitum chow-fed groups. Quantitation of the stages of seminiferous tubules or the number of homogenization-resistant, mature spermatids per testis were not affected by 6-MP treatment. In addition, 6-MP had no effect on serum testosterone or on HCG-stimulated testosterone release by the testes. These results suggest that chronic low-dose 6-MP therapy, as used in the treatment of IBD, may not carry as great a risk for suppression of spermatogenesis as theorized. Our study in animals indicates that evaluation of 6-MP and spermatogenesis in man is warranted.