1 The effects of selective and non-selective beta-adrenoceptor blockade upon plasma non-esterified fatty acid concentrations in the fasting state and following insulin stress have been studied in normal subjects. 2 Atenolol, propranolol and placebo were compared in a double-blind cross-over trial in eight normal subjects. 3 Atenolol and propranolol significantly lowered plasma non-esterified fatty acid concentrations by a similar degree in the fasting, non-stressed state. This finding suggests that beta1-adrenoceptors are involved in the control of basal lipolysis. 4 Following insulin-induced stress, lower plasma non-esterified fatty acid concentrations were observed with propranolol than with atenolol. This difference may be due to beta2-adrenoceptor involvement in the stress mechanisms controlling lipolysis, or to the differences in the water-lipid solubility properties of these drugs.