1. Using whole-cell and single channel recordings, we have examined the properties of acetylcholine (ACh)-activated currents in neurones from larval and pupal Drosophila melanogaster (fruit fly), larval and embryonic Musca domestica (house fly), and nymphal Schistocerca gregaria (locust). 2. In all preparations, single channel recordings revealed two major classes of ACh-activated channels, with average conductances of approximately 32 and 59 pS. 3. At ACh concentrations from 1 to 10 microM, channel activity in Drosophila larval neurones occurs in bursts with an average of 1-2 openings. Open times and burst durations are described by one or two exponentials. Burst durations for the 32 pS channel (approximately 3 ms, slow component) were longer than those for the 59 pS channel (approximately 1.0 ms). The mean open interval duration for the 32 pS channel (slow component) was also longer than that of the 59 pS channel. 4. At high ACh (20-200 microM) concentrations, bursts of the smaller conductance channel occur in clusters separated by long-lived periods without channel activity. Considerable kinetic heterogeneity was observed among clusters. 5. The whole-cell dose-response curve suggests that activation of current by ACh increases up to at least 100 microM and that multiple ligand binding steps are involved. 6. Drosophila and Musca larval neuronal ACh-activated channels show some unique features in their cholinergic pharmacological properties: (a) they are only weakly activated by the potent neuromuscular nicotinic agonist suberyldicholine, (b) hexamethonium and decamethonium are weak, but approximately equi-effective blockers, and (c) alpha- and kappa-bungarotoxin (BTX) both blocked reversibly, though alpha-BTX appears to be the more potent inhibitor.