Dopamine and apomorphine have been compared with regard to contraction and relaxation of aortic strips prepared from rats of different ages. Both dopamine and apomorphine contracted aortic strips from older (9-12 weeks) rats to greater maximal tension than preparations from younger (3--5 weeks) animals. Contraction in response to both agonists could be blocked by the alpha-blocker, Dibenamine. In contrast to dopamine contraction by apomorphine was associated with the development of tachyphylaxis. Both apomorphine and dopamine relaxed the aorta after alpha blockade, however, the relaxation produced by these drugs differed in two major aspects. First, dopamine-induced relaxation was greatest in aortic strips from young rats compared to older rats whereas relaxation with apomorphine was not age dependent. Second, dopamine-induced relaxation was abolished by propranolol but not haloperidol whereas apomorphine relaxation was only blocked by haloperidol. These data establish that the aortic relaxation caused by dopamine is most likely a beta-adrenergically mediated response whereas that produced by apomorphine is not. Moreover, only dopamine was able to increase the concentration of rat aortic cyclic AMP. Should these data be applicable to other vascular beds or species, it is possible that the vascular effects of dopamine will be influenced by age and drugs which impinge on cyclic nucleotide disposition.