Increases in cyclic adenosine monophosphate (cAMP) are proposed to initiate learning in a wide variety of species. Here, we measure changes in cAMP in the olfactory bulb prior to, during, and following a classically conditioned odor preference trial in rat pups. Measurements were taken up to the point of maximal CREB phosphorylation in olfactory bulb mitral cells. Using both drug and natural unconditioned stimuli we found effective learning was associated with an increase in cAMP at the end of the conditioning trial, followed by a decrease 5 min later. This early timing of a transient cAMP increase occurred only when the odor was paired with an effective drug or natural unconditioned stimulus (US). The data support the hypothesis that the rate of adenylate cyclase activation is enhanced by pairing calcium and G-protein activation and that the timing of transient cAMP signaling is critical to the initiation of classical conditioning.