Effects on plasma T, DHT, A, FSH, LH and PRL concentrations and the pituitary responsiveness to the LHRH/TRH stimulation, as well as on libido and sexual, mental and physical activities were studied in ten hypogonadal male patients undergoing therapy with oral testosterone undecanoate (TU) for 9 weeks. The daily dose of TU needed for satisfactory clinical effect was 120 mg (6 cases) and 240 mg (4 cases). There was a significant rise in circulation androgen concentrations in all patients. Mean plasma T, DHT and A increased at 9 weeks from 1.20, 0.12 and 0.36 ng/ml to 4.34 (P less than 0.0004), 0.39 (P less than 0.0004) and 1.24 ng/ml (P less than 0.005), respectively. In hypergonadotrophic patients (N = 6) plasma FSH and LH fell progressively (P less than 0.05), while in hypogonadotrophic patients (N = 4) a marked rise in plasma FSH (P less than 0.05) was found, while LH tended to rise as well. Base-line plasma PRL remained unchanged. In three out of four patients with poor PRL response to TRH, normal responses were established after TU therapy. Increase in libido and sexual activity was reported by nine patients. An increase in mental and physical activity was found in seven and two patients, respectively. Tolerance was excellent. It was concluded that oral TU is an effective form of substitution therapy in male hypogonadal patients.