Abstract: Plasm testosterone（T）,LH and FSH were measured in thirteen hypogonadal men before and, at hourly intervals,after the oral administration of 80 mg testosterone undecanoate（TU）.The test was repeated in one man after he had taken 120 mg TU per day for eight weeks.The sexual interests and behavioural effects were studied in 4 hypogonadal male patients during eightweek administration of TU.Plasma T increased gradually,reached the xenith（22.2±3.4nmol/l）four hours after TU administration and returned to the basal level at the tenth hour.However the absorption curves showed a high interindividual variability in the time when maximum concentrations were reached,as well as in the maximum levels themselves.The increased LH and FSH in hypergonadotropics（N=5）were not suppressed.In one patient the long-term treatment made the peak occur earlier.The sexual interests and erections as well as physical activity were improved in all four patients who had taken TU for eight weeks.No side effects were complained.The convenience of oral administration,the resulting prolonged elevated plasma androgen levels and the probable lack of deleterious effects on the liver apparently render oral TU of value when androgen replacement therapy is indicated.