Choline acetyltransferase (ChAT) activity, acetylcholinesterase (AChE) activity and muscarinic cholinergic receptor binding were determined in homogenates of olfactory bulbs from rats killed at intervals from 4 days before through 60 days after birth. In addition, the localization of muscarinic receptors was determined using an in vitro autoradiographic technique in 6-millimicrons thick coronal sections of olfactory bulbs from rats killed at similar intervals after birth. All 3 cholinergic parameters were present in measurable quantities at birth and showed substantial increases between 1 and 20 days after birth. The most rapid increase in cholinergic parameters occurred between days 10 and 20 after birth. ChAT activity and muscarinic receptor binding decreased between days 20 and 35 and increased again between postnatal days 35 and 60. A similar developmental pattern was observed for autoradiographic grain density overlying the granule cell layer of the neonatal bulb. These data suggest that (1) centrifugal cholinergic afferents are present in the rat olfactory bulb at birth, (2) during the early postnatal period (between 10 and 20 days) synaptogenesis occurs resulting in an overproduction of cholinergic synapses and (3) between postnatal days 20 and 35, a period of synaptic reorganization occurs characterized by substantial regression.