The action of a synthetic 'atrial natriuretic factor' (sANF) on induced tone in isolated rat renal resistance vessels (lumen diameter about 200 microns) was examined and compared with the effects of sANF on resistance vessels of similar size taken from the cerebral, mesenteric and femoral vasculature. Synthetic ANF caused a relaxation of the renal vessels when these were submaximally activated with noradrenaline or serotonin, but had no effect on the responses of the other vessels to these agonists. In contrast to previous reports concerning rabbit aortic vessels, methylene blue (which is thought to cause inhibition of guanyl cyclase) did not reduce the dilator response to sANF in the renal vessels. The results demonstrate that sANF has a specific relaxing effect on renal resistance vessels, and are consistent with its effect being mediated through specific receptors. The mechanism of this relaxant effect remains unknown.