In an effort to understand the developmental changes in the distribution of muscarinic receptor subtypes (m1-m5), specific brain regions from juvenile (16-day-old), young (21-day-old) and adult (90-day-old) rats were analyzed using subtype-selective antibodies. These studies revealed significant age-dependent changes in the four brain regions examined. In cortex, an area associated with higher cognitive functions, significant increases of m2 and m4 receptors occurred between juvenile and adult rats. In the striatum, the level of m4 receptor increased with age whereas the m1, m2 and m3 receptors had reached mature levels within the first 16 days. Small but significant changes occurred in the cerebellum with a decrease in m1, m3 and m4 receptor subtypes. In contrast to other brain regions, the hippocampus displayed consistent expression levels of muscarinic receptor subtypes. Suggesting that this brain region, which is involved in the foundation of numerous neural networks, requires a full complement of muscarinic receptors at a very early age. Muscarinic receptors have been shown to be important in a number of behavioral activities, including learning and memory. The changes observed in the age-dependent expression of these receptors most likely play an important role in how acetylcholine produces its effects in vivo.