Oral yohimbine administration (0.2 mg/kg) induced lipid mobilization (increase in plasma non-esterified fatty acids, NEFA) in fasting non-obese women (body mass index BMI = 20.2 +/- 0.5, age 35.5 +/- 2.7 years) without significant action on plasma glucose, insulin levels, heart rate or blood pressure during the time-course of the experiment (240 min). Plasma norpinephrine (but not epinephrine) concentrations were increased (100 percent) after oral yohimbine administration. Oral administration of propranolol (40 mg, 60 min before yohimbine) reduced the lipid-mobilizing action of yohimbine (70 percent) during the 60 min following its administration and then totally suppressed its effect until the end of the experimental period (180 min). In fasting obese women (BMI = 36.4 +/- 2.1, age 37 +/- 3.6 years), yohimbine provoked an increase in plasma NEFA levels which was not markedly different from that observed in non-obese subjects. It had no significant effect on plasma glucose, insulin levels, heart rate or blood pressure. Plasma norepinephrine increased in the same proportions. The lipid-mobilizing effect of yohimbine in women is mainly attributable to the increase in synaptic norepinephrine with a resultant increment in lipolysis by beta-adrenergic agonism. In the standard fasting conditions (12 hours) the blockade of the antilipolytic fat cell alpha 2-adrenoceptors seems to be a minor component of the lipomobilizing effect of yohimbine. Morever, when compared with non-obese women, the lipomobilizing effect of yohimbine is not enhanced in obese women.