Pertussis toxin, a substance known to inactivate the inhibitory guanine nucleotide regulatory unit of adenylate cyclase, was injected into the lateral cerebral ventricle of rats; intracellular recordings were made from locus coeruleus neurons in brain slices 1-3 days later. Morphine (an opiate agonist) and clonidine (an alpha 2-agonist) produced the expected outward currents (and associated hyperpolarization and inhibition of firing) in controls whereas the effects of both agonists were blocked in animals pretreated with pertussis toxin. These results are consistent with the hypothesis that opiate and alpha 2-agonists may depress the firing of locus coeruleus neurons by inhibiting adenylate cyclase via a guanine nucleotide regulatory protein.