1. The nature of the rat epididymal adipocyte beta-adrenoceptor was investigated by studying the effects of beta 1- and beta 2-selective antagonists on lipolysis induced by (-)-isoprenaline and the lipolytically selective agonist BRL 37344. 2. From 10 nM to 10 microM, the potent and highly selective beta 1-adrenoceptor antagonist CGP 20712A did not influence the concentration-response curve (CRC) of BRL 37344 whereas small but consistent shifts to the right of the (-)-isoprenaline-induced CRC were observed. Clear rightward shifts of the CRCs induced by both (-)-isoprenaline and BRL 37344 were produced only at 100 microM CGP 20712A with the corresponding pA2 values being 4.80 and 4.61, respectively. 3. When the beta 2-selective antagonist ICI 118,551 was used at 10 microM and higher, clear and concentration-dependent shifts to the right of the CRCs of both agonists were observed. The slopes of the Schild plots did not deviate significantly from unity, the pA2 values being 5.49 and 5.33 against (-)-isoprenaline and BRL 37344, respectively. 4. The results demonstrate that (-)-isoprenaline-induced lipolysis in rat white adipocytes is mediated predominantly by atypical beta-adrenoceptors, whereas the typical beta 1-adrenoceptors play a small, subordinate role. The lipolytically selective agonist BRL 37344 acts solely through atypical beta-adrenoceptors.