Oestradiol administration in vivo has been shown to potentiate the adrenergic lipolysis in vitro in rat epididymal adipose tissue; one of the possible explanations of this oestradiol effect might be the direct influencing of beta-adrenergic receptors. In order to show this possible mechanism of action, in direct radioligand binding studies we have estimated the specific binding parameters of beta-adrenergic receptors in controls and in vivo pretreated rats by a single dose of oestrogen (200 microgram.kg-1 s.c. 24 hours prior sacrification of rats). Beta-adrenergic receptor binding studies we performed with membrane fraction of homogenates of epididymal adipose cells, the radioligand used was L-[3H]-dihydroalprenolol (3H-DHA). No clear-cut differences in the saturation curves of 3H-DHA were found between control and pretreated oestradiol rats; the density of beta-adrenoreceptors (Bmax) was practically identical (269 v. 270 fmol.mg-1 protein) and also differences in dissociation constants (Kd) of both groups were minimal (1.05 vs. 0.95 nmol.1-1). Minimal differences between control and oestradiol pretreated rats were also found in displacement studies, where the effects of 1-isomers of norepinephrine and epinephrine was tested. Thus, it seems possible that the potentiating effect of in vivo pretreatment by oestradiol on adrenergic lipolysis of epididymal adipose tissue in vitro is not caused by the direct influencing of beta-adrenergic receptors. The explanation of this effect will be necessary to search on other steps of the adrenergic reaction which lead to lipolysis or in influencing other receptors (e.g. alpha-adrenergic receptors).