1. Dose-dependent vasodilator responses to dopamine, isoprenaline, noradrenaline, 3-isobutyl-1-methylxanthine (IBMX) and sodium nitroprusside were obtained in isolated perfused mesentery preparations, taken from reserpine-treated rats of different ages. The preparations were pretreated with phenoxybenzamine (1 microM) and perfused with physiological salt solution containing cocaine (10 microM), additional KCl (20 mM) and vasopressin (0.1 microM). 2. Vasodilator responses to dopamine were abolished by the dopamine1 (DA1)-selective antagonist SCH 23390 (10 nM) and those to isoprenaline by propranolol (1 microM), but the vasodilator responses to noradrenaline were abolished only when SCH 23390 and propranolol were used together. This indicated that dopamine was acting via DA1-receptors, isoprenaline via beta-adrenoceptors and that noradrenaline could act via DA1-receptors and beta-adrenoceptors in this preparation. 3. Responses to all the vasodilator drugs decreased in magnitude between the ages of 1 and 2 months. Responses to dopamine declined further in 4 month-old rats and were negligible at 6 or 22-24 months of age. Responses to isoprenaline were well maintained up to 6 months of age, but were negligible at 22-24 months. 4. It is concluded that, in the rat mesenteric vasculature, there is a non-specific decline in responses to vasodilator drugs during development (1 to 2 months). Subsequently there is a specific decline in DA1-receptor-mediated and beta-adrenoceptor-mediated responses; the former are lost at an earlier age than the latter. This different time course suggests that age influences receptor numbers, or their coupling to adenylate cyclase, rather than a post-receptor event in the adenylate cyclase/cyclic AMP pathway.