Cyclic 3’,5’-adenosine monophosphate (cAMP) accumulation and morphological changes induced by isoproterenol (ISO) on cultured vascular smooth muscle cells (SMC) and vascular fibroblasts derived from spontaneously hypertensive rats, their stroke-prone strain and normotensive Wistar Kyoto rats were investigated. At the time points studied, ISO-induced cAMP accumulation in SMC reached a peak level at 5 min. Accumulation was dose-dependent and was maximal at a concentration of 10<sup>–5</sup> M ISO. Maximal cAMP levels were approximately 600-fold higher than basal levels. Maximal cAMP accumulation or half maximal stimulatory ISO concentrations were similar in SMC from the three strains. ISO had no effects on cyclic 3’,5’-guanosine monophosphate (cGMP) levels in SMC. Phenylephrine had no effects on cAMP or on cGMP levels. In contrast to SMC, β-adrenergic stimulation of vascular fibroblasts resulted in only a 4-fold increase of cAMP levels. 1.5 h after administration of ISO to SMC cultures, the morphological changes were apparent in SMC but not in fibroblasts. Morphological changes induced by ISO were reversible and morphological appearances returned to normal 16 h after exposure to ISO. 10<sup>-3</sup> M dibutyryl cAMP had similar effects on the moφhologies of both SMC and fibroblasts. These effects were antagonized by 5 × 10<sup>–6</sup> M colchicine, an inhibitor of microtubule assembly. These results indicate that cultured vascular SMC possess the ability to increase markedly their cellular cAMP level in response to β-adrenergic stimulation, while fibroblasts are less responsive to the stimulation. Furthermore, cAMP accumulation results in morphological changes of SMC and fibroblasts probably through the alteration of intracellular microtubule systems. As the morphological response to intracellular cAMP (or its lipophilic derivatives) is similar in both SMC and fibroblasts, the difference in each cell line’s responsiveness to ISO may be due to a difference in: (1) the density or sensitivity of β -adrenergic receptors on the plasma membrane of each cell type, or (2) the catalytic activity of adenylate cyclase itself.