A method is described for the quantification of vascular smooth muscle cell growth from individual explants of contractile rabbit aortic tunica media. The precision of the method probably depends on regular explant geometry (1-mm squares) and pooling sufficient explants. Serum-induced growth was quantified by measurements of ATP concentration, incorporation of [3H]thymidine and DNA concentration. The possible effects of endogenous vasodilator agents on growth were investigated by using lipid soluble analogues of their second messengers, namely 8-Br-cAMP and 8-Br-cGMP, which are known to relax rabbit aortic strips. Cell growth was inhibited concentration-dependently by 8-Br-cAMP but not 8-Br-cGMP (0.01-1 mM). The effect of 8-Br-cAMP was reversible, and also occurred when addition was delayed until after growth had commenced. The results imply that endogenous vasodilators such as prostacyclin, adenosine and adrenaline, which increase cAMP concentration, may normally suppress smooth muscle cell growth, whereas nitric oxide and atriopeptins, which increase cGMP concentration, may not.