The function of the secondary sex organs was found to be markedly impaired in 29 participants in a methadone maintenance program. The ejaculate volume and seminal vesicular and prostatic secretions were reduced by over 50 per cent in methadone clients, as compared to 16 heroin addicts and 43 narcotic-free controls. Serum testosterone levels were also approximately 43 per cent lower in methadone clients than in controls or heroin users. Although the sperm count of methadone clients was more than twice the control levels, reflecting a lack of sperm dilution by secondary-sex-organ secretions, the sperm motility of these subjects was markedly lower than normal. On all measures of secondary-sex-organ and testicular function, heroin addicts appeared to fall between the methadone and control subjects, but, with the exception of sperm motility, the deviation from control values did not reach statistical significance. (N Engl J Med 292:882–887, 1975), RECENT evidence obtained in our laboratories indicates that chronic morphine or methadone administration produces a pronounced reduction in the size and secretory activity of the male rat's secondary sex organs.