Recent studies in men have shown that abdominal fat increases with age and decreasing testosterone concentrations. Furthermore, in cell culture, testosterone expresses an increased lipolytic potential and depresses lipoprotein lipase activity (LPL) in adipose cells. These metabolic characteristics are found in abdominal adipose tissue in young men. In order to see whether abdominal fat masses in moderately obese middle-aged men might be diminished by testosterone, this hormone was given either as a single injection (500 mg) or in moderate doses (40 mg X 4) for 6 weeks in an oral preparation, bypassing the liver. When measured 1 week after the single dose, abdominal LPL tended to decrease. After 6 weeks a dramatic decrease of abdominal LPL was found, as well as an increase in the lipolytic responsiveness to norepinephrine, both changes confined solely to the abdominal, and not femoral adipose tissue regions. The waist/hip circumference decreased in 9 out of the 11 examined men. No untoward effects were seen in behavioural variables, blood pressure, triglyceride or cholesterol values, and liver function tests. These preliminary results suggest that administration of testosterone in moderate doses to middle-aged men lead to adaptations of the metabolism of adipose tissue expected to be followed by a diminution of this mass.