Relaxation of smooth muscle cells induced by activation of beta-adrenoceptors was investigated in intact and skinned muscles of the guinea-pig mesenteric artery.1. In concentrations over 10(-7) M, isoprenaline reduced the resting tone of intact preparations and also the amplitude of K contractions. When Ca was applied after previous superfusion with Ca-free solution, the amount of Ca accumulated into storage sites was increased by isoprenaline in polarized and depolarized ([K](o) 128 mM) muscles. The amount of Ca stored increased even further when procaine and isoprenaline were applied simultaneously during store loading.2. Isoprenaline increased the concentration of cyclic AMP as determined by radioimmunoassay. Application of isoprenaline at a concentration of 10(-7) M increased cyclic AMP from 2.2+/-0.3 to 2.8+/-0.6 p-mole/mg wet weight and at 10(-6) M increased it to 4.5+/-0.8 p-mole/mg wet weight after 5 min incubation (n = 4).3. Application of cyclic AMP (3 x 10(-6) M) with cyclic AMP-dependent protein kinase (50 mug/ml.) had no effect on the pCa-tension relationship in the skinned muscles. However, an increased concentration of cyclic AMP (> 10(-5) M) suppressed the Ca-induced concentration only in the presence of protein kinase. This protein kinase (50 mug/ml.) alone had no effect on the Ca-induced contraction.4. In skinned fibres, the Ca store could be loaded by applying low concentrations of Ca. If cyclic AMP (3 x 10(-6) M) with protein kinase (50 mug/ml.) was applied during the loading procedure, the amount of Ca accumulated by the store increased if the loading solution contained 10(-6) M-Ca applied for 2 min or less, but if the loading solution was applied for 3 min, or if higher Ca concentrations were used, the presence of cyclic AMP with protein kinase decreased the store size, suggesting that a Ca-induced Ca-release mechanism was also being activated.5. In skinned muscles, accumulation of Ca into the store site in the presence of cyclic AMP (3 x 10(-6) M) with protein kinase (50 mug/ml.) was further accelerated by simultaneous applications of procaine (5 mM), as here the Ca-induced Ca-release mechanism was suppressed.6. These results indicate that activation of beta-adrenoceptors by isoprenaline increases the amount of cyclic AMP in the intact muscles, and leads to an increase in Ca accumulation into the store site. In the skinned muscles, the Ca-induced Ca-release mechanism is activated by cyclic AMP and the Ca receptor for contraction (leiotonin C or calmodulin) is somewhat suppressed. These effects of exogenously applied cyclic AMP require the presence of protein kinase. The relaxation following beta-adrenoceptor activation is more likely to involve Ca extrusion from the cell and accumulation of Ca in internal storage sites than suppression of the binding of calmodulin with the myosin light chain kinase.