In order to study human fat cell morphology and metabolism in both sexes, with different levels of fatness and physical training, 53 subjects (14 women and 39 men), 25.9 +/- 7.6 (mean +/- SD) years of age, weighing 64.8 +/- 10.8 kg and with 14.8 +/- 4.7% body fat were submitted to a biopsy of subcutaneous adipose tissue in the supra-iliac region. Average fat cell diameter (ACD) was assessed by measuring at least 500 cells per subject. Basal (BL), epinephrine sub-maximal (10(-5) M) (ESML) and epinephrine maximal (10(-4) M) (EML) stimulated lipolysis have been determined on collagenase isolated fat cells. Female subjects have larger ACD (83.4 +/- 12.3 micrometer vs. 62.5 +/- 11.9 micrometer) (P less than or equal to 0.001) and lower BL, ESML, and EML than men (P less than or equal to 0.05). After control over age and sex of subjects, there were significant and negative correlations between all fatness indicators (ACD, fat mass in kg, percent body fat, sum of 9 skinfolds) and BL, ESML and EML. A completely randomized factorial ANOVA design revealed a significant effect of fatness level on BL, ESML and EML, but a non-significant trend for the effect of physical training on the same variables. It is concluded that lipolytic activity is more related to body fatness than to the training status. However, training tends to accentuate lipolytic activity as shown by values of BL, ESML and EML found in the lean highly trained subjects of the study.