The mechanism by which carbamate pesticides inhibit rat alpha4beta2 nicotinic acetylcholine (ACh) receptors (nAChRs) expressed in Xenopus laevis oocytes has been investigated using the two-electrode voltage clamp technique. Carbaryl, S-ethyl N,N-dipropylthiocarbamate (EPTC), and fenoxycarb inhibit ACh-induced ion currents in a concentration-dependent way. EPTC and fenoxycarb inhibit ion currents induced by 1 mM ACh with 3-fold to 5-fold higher potency than ion currents induced by 1 microM ACh. The potency of carbaryl appears to be independent of ACh concentration. Fenoxycarb displaces (3)H-epibatidine bound to alpha4beta2 (nAChRs) with a K(i) of 750 microM, which is much higher than the functional IC(50) of 2.3-11 microM. This shows that the inhibition of ion current by the carbamate is a noncompetitive effect. Inhibition by fenoxycarb is independent of the state of the ion channel. The rate of onset of inhibition is enhanced, and the rate of reversal of inhibition is reduced, when the concentration of fenoxycarb is increased. The rate of reversal of inhibition is also reduced when the period of exposure to fenoxycarb is increased. The time- and concentration-dependent inhibition of nAChR-mediated ion current by fenoxycarb is accounted for by a two-step mechanism involving a rapid blocked state and a sequential more stably blocked or desensitized state.