The exocytotic release of potent hormones is a tightly controlled process. Its direct regulation without the involvement of second messengers would ensure rapid signal processing. In streptolysin O-permeabilized insulin-secreting cells, a preparation allowing dialysis of cytosolic macromolecules, activation of alpha 2-adrenergic receptors caused pertussis toxin-sensitive inhibition of calcium-induced exocytosis. This inhibition was mimicked very efficiently by the use of specific receptor-mimetic peptides, indicating the involvement of Gi and, to a lesser extent, of G(o). The regulation was exerted beyond the ATP-dependent step of exocytosis. In addition, low nanomolar amounts of pre-activated Gi/G(o) directly inhibited exocytosis. As transient overexpression of constitutively active mutants of G alpha i1, G alpha i2, G alpha i3 and G alpha o2 but not of G alpha o1 reproduced this regulation, the G alpha subunit alone is sufficient to induce inhibition. These results define exocytosis as an effector for heterotrimeric G-proteins and delineate the properties of the transduction pathway.