The objective of this work was to study adrenoceptor sensitivity in vivo in a number of tissues in lean and obese humans. The thermogenic, metabolic and cardiovascular responses to a 90 min infusion of adrenaline were measured. The subjects were eleven obese subjects (Body Mass Index 36.0 +/- 1.2 kg/m2) and 10 non-obese subjects (Body Mass Index 21.9 +/- 0.7 kg/m2). Metabolic rate, heart rate, blood pressure, forearm blood flow, plasma palmitate turnover and oxidation were measured. Thermogenic responses to adrenaline were similar in the lean and obese groups (14.4 +/- 1.6 and 15.1 +/- 1.6 J/min/kg fat free mass respectively). Of the cardiovascular variables measured, only the increase in forearm blood flow during adrenaline infusion differed between lean and obese, being 3.9 +/- 0.5 and 1.9 +/- 0.3 ml/min/100 ml forearm respectively. Basal plasma palmitate turnover rates were lower in the obese when expressed per unit fat mass (2.32 +/- 0.17 and 7.61 +/- 1.20 mumol/min/kg fat mass respectively). Basal plasma palmitate oxidation rates were higher in the obese when expressed per unit fat free mass (1.53 +/- 0.19 and 0.82 +/- 0.12 mumol/min/kg fat free mass respectively). In response to adrenaline palmitate turnover increased similarly in both groups, but plasma palmitate oxidation rates fell in the obese whilst they were unchanged in the lean. In the basal state the obese do not appear to have a defect in fat oxidation, but their response to infused adrenaline may favour fat storage over oxidation. No thermogenic defect was shown in the obese.