To investigate the role of actin in vertebrate nerve terminals, nerve-muscle preparations from garter snake (Thamnophis sirtalis) were treated with the actin-depolymerizing agent latrunculin A. Immunostaining revealed that actin filaments within presynaptic motor terminal boutons were disrupted by the drug. In preparations loaded with the optical probe FM1-43, destaining was reduced by latrunculin treatment, suggesting that transmitter release was partially blocked. Latrunculin treatment did not influence the amplitude or time course of spontaneous miniature endplate potentials (MEPPs). Similarly, endplate potentials (EPPs) evoked at low frequency were comparable in control and latrunculin-treated curarized preparations. Brief tetanic stimulation of the muscle nerve (25 Hz, 90 s) depressed EPP amplitudes in both control and latrunculin-treated preparations. After tetanus, EPPs elicited at 0. 2 Hz in control preparations recovered rapidly (0-5 min) and completely (usually potentiating to above pre-tetanus levels; 130 +/- 11 %, mean +/- s.e.m.). In contrast, EPPs evoked in latrunculin-treated preparations recovered slowly (8-10 min) and incompletely (84 +/- 8 %). The influence of latrunculin on post-tetanic EPPs depended on its concentration in the bath (KD = 3. 1 microM) and on time of incubation. These observations argue that actin filaments facilitate transmitter release rather than impede it. Specifically, actin may facilitate mobilization of vesicles towards the releasable pools.