We studied the ontogeny of muscarinic depression in the developing rat visual cortex using carbachol (a nonhydrolyzable cholinergic agonist) application to neocortical slices obtained from four postnatal age groups: 9-10 days, 15 days, 30-40 days and 18 months. Carbachol suppressed the evoked synaptic response of layers II-III to stimulation of layer II-III afferents. Atropine eliminated the carbachol effect, suggesting that it is mediated by muscarinic receptors. The results indicate a significant increase in muscarinic efficacy in the developing rat visual cortex.