Evidence was sought for the existence on cholinergic nerve terminals in guinea-pig atria of alpha-adrenoceptors subserving inhibition of acetylcholine release. The experiments were performed with atria which had been incubated with 3H-choline and transmitter release was deduced from the efflux of radioactivity elicited by field stimulation. In preparations which had been incubated with 3H-choline, field stimulation (60 pulses, 2 Hz) evoked release and radioactivity which was inhibited by 1.0 mumol/l noradrenaline, in the presence of propranolol (1.0 mumol/l), but was unaltered by clonidine (1.0 and 10.0 mumol/l). The inhibitory effect noradrenaline on the stimulation-induced efflux of radioactivity was blocked by idazoxan (0.3 mumol/l), and phentolamine (1.0 mumol/l) but not by prazosin (0.3 mumol/l). In the presence of propranolol (1.0 mumol/l), neither phentolamine (1.0 mumol/l), idazoxan (0.3 mumol/l) nor prazosin (0.3 mumol/l) had any effect on stimulation-induced efflux of radioactivity. Stimulation of the extrinsic vagus nerve of atrial preparations with trains of pulses at frequencies of 2, 4, 8, and 16 Hz produced graded decreases in the rate of atrial beating. The negative chronotropic responses to vagus stimulation were unaffected by noradrenaline (1.0 mumol/l) in the presence of propranolol (1.0 mumol/l). These findings indicate that the release of acetylcholine from the cholinergic terminals in guinea-pig atria can be inhibited by a mechanism apparently involving prejunctional alpha 2-adrenoceptors. However, under the experimental conditions used here the chronotropic responses of atria to stimulation of the extrinsic vagus nerve was not affected by activation of the prejunctional alpha 2-adrenoceptors associated with the cholinergic terminals.