The effects of androgen withdrawal and replacement were investigated in six hypergonadotrophic and six hypogonadotrophic men with hypogonadism. A double blind cross-over design was used comparing testosterone undecanoate (T.U., Organon International), 160 mg daily by mouth, with placebo. There was a significant effect of T.U. on all measures of sexual interest and behaviour. Sexual interest increased within the first week of T.U. administration, ejaculation usually returning within the second week. Behavioural responses were similar in the hyper- and hypogonadotrophic groups. The rise in plasma testosterone during T.U. administration was modest, not reaching the normal range in several cases. 5 alpha dihydrotestosterone rose more substantially. There was a significant fall in sex hormone binding globulin, a rise in oestradiol and no significant change in gonadotrophin concentrations during T.U. administration.