Gamma-Aminobutyric acid-B (GABAB) receptors mediate a variety of cellular functions, suggesting the possibility of pharmacologically and molecularly distinct receptors. To explore this possibility a number of GABAB receptor agonists and antagonists were examined for their ability to influence cAMP production in rat brain cerebral cortical slices. While the agonists did not differentiate between receptors associated with the augmentation of isoproterenol-induced cAMP production and those mediating inhibition of forskolin-stimulated second messenger accumulation, significant differences were noted between the potencies of some antagonists to inhibit these GABAB receptor-mediated responses. The results suggest at least two pharmacologically distinct subclasses of GABAB receptors regulate cAMP production in brain.