The ontogeny of muscarinic acetylcholine receptors in the rat brain has been examined using the radioligands, [3H]N-methylscopolamine, [3H]propylbenzilylcholine and [3H]oxotremorine-M. In the 3 regions of the brain selected for study, the cerebral cortex, the diencephalon and the medulla-pons, the receptors develop at different rates. The most rapid development takes place in the medulla with considerably slower maturation in the diencephalon and cerebral cortex. In the cortex, the agonist binding properties of the muscarinic receptors vary during development. There appears to be a 6--7 day lag in the appearance of high affinity sites following formation of low affinity sites.