Histamine, acting via H1 receptors, augments adenosine-induced cAMP accumulation in slices of guinea pig cerebral cortex by an indirect mechanism that appears to involve an intracellular messenger(s). To better characterize this process, the time course of this augmentation was examined in slices prelabeled with [3H] adenine. When histamine (1 mM) was added after the cAMP level had reached steady state with adenosine (0.1 mM), the cAMP level rose to a new steady level within 10 min (t 1/2, 2-3 min). There was no measurable delay in this response, indicating rapid activation of the augmentation after receptor occupation. Studies using the H1 receptor antagonist mepyramine indicated that the continued presence of the histamine stimulus was required to maintain the augmentation. Addition of mepyramine (10 microM) between 1 and 14 min after histamine caused cAMP levels to fall to a level similar to that obtained previously with adenosine alone, but with a delay of 2-3 min. This gives an upper estimate of the lifetime of any intracellular messenger involved in the augmentation process. To determine whether histamine acts by stimulating synthesis of cAMP or by inhibiting its breakdown, the fall in tissue cAMP content was studied after rapid removal of the adenosine stimulus by addition of adenosine deaminase. The initial fall was significantly faster in slices incubated with 0.1 mM adenosine plus 1 mM histamine than in slices with 0.1 mM adenosine alone, indicating increased synthesis and breakdown of cAMP in the presence of histamine. However, the higher breakdown rate probably reflects stimulation of the degradation process by the higher initial level of cAMP with histamine because, at equivalent levels, cAMP content fell at similar rates in both conditions. This was confirmed in other experiments in which similar steady state cAMP levels were achieved with and without histamine by appropriate choice of adenosine concentrations. It is therefore concluded that the direct effect of histamine is primarily to potentiate cAMP synthesis.