The relationship between the density of the muscarinic receptor in developing rat cerebral cortex and its coupling to phosphoinositide turnover is examined. Tissue slices from rats of various ages were incubated with myo-[2-3H]inositol, and the effect of carbamoylcholine on the release of total inositol phosphates was determined. Binding of [3H]quinuclidinyl benzilate was determined in the same tissue. Although muscarinic receptor density in day-18 embryonic cortex was only 5% of that in the adult, the maximal response of stimulated phosphoinositide turnover to carbamoylcholine (1-10 mM) was at the adult level (i.e., three-fold increase). Comparison of the dependence of the turnover on carbamoylcholine concentration revealed that in neonates, the dose-response curve was shifted to the left, giving a half-maximal effect at concentrations approximately tenfold lower than that in the adult. In addition, the partial muscarinic agonists oxotremorine-2 and bethanechol were both more efficacious in young rats than in adults. The differences could not be accounted for either by alterations in agonist affinity for the receptor or by the presence of "spare" muscarinic receptors. These results indicate that muscarinic receptors in fetal and newborn rat cerebral cortex are more efficiently coupled to stimulation of phosphoinositide turnover than in the adult.