1. The effect of the spider venom argiopine on L-glutamate-activated membrane channels of crayfish muscle was investigated using the patch-clamp technique. 2. When 10(-2) M-glutamate and 10(-9) M-argiopine were contained in the pipette solution of a cell-attached patch, bursts of openings of excitatory channels appeared after formation of the patch. These bursts ceased abruptly after variable periods of time in the range of 5 min. Higher concentrations of argiopine (up to 10(-6) M) blocked more rapidly, approximately in proportion to concentration. 3. The block of excitatory channels could be partially or completely reversed by hyperpolarizing the membrane by up to -190 mV from the resting potential. The time constant of the recovery of channel opening decreased with increasing hyperpolarization and was 2 ms with -160 mV hyperpolarization. Switching back from the hyperpolarized level to the resting potential, the time constant for the resulting block was about 3 s (10(-7) M-argiopine). Potential-dependent block by argiopine with similar characteristics was also observed in outside-out patches. 4. Up to argiopine concentrations of 10(-7) M the kinetics of channel openings and of bursts measured in pre-block periods or during reversal of the block by hyperpolarization were indistinguishable from controls. 5. When the potential-dependent block observed in the presence of 10(-6) M-argiopine and 10(-2) M-glutamate was reversed by hyperpolarization, additional short closings occurred during bursts. This 'flickering block' did not change burst length appreciably, but an additional open time component (tau = 0.1 ms) appeared and the average open time per burst was reduced. 6. At least two reaction steps seem necessary to model the behaviour of the potential-dependent block. The flickering block may be described as intermittent blocking of the channel which does not interfere with the reactions between glutamate and the channel.