1 Ciguatoxins (CTXs) are known to bind to receptor site 5 of the voltage-dependent Na channel, but the toxin's physiological effects are poorly understood. In this study, we investigated the effects of a ciguatoxin congener (CTX3C) on three different Na-channel isoforms, rNa v 1.2, rNa v 1.4, and rNa v 1.5, which were transiently expressed in HEK293 cells. 2 The toxin (1.0 mmol l À1 ) shifted the activation potential (V 1/2 of activation curve) in the negative direction by 4-9 mV and increased the slope factor (k) from 8 mV to between 9 and 12 mV (indicative of decreased steepness of the activation curve), thereby resulting in a hyperpolarizing shift of the threshold potential by 30 mV for all Na channel isoforms. 3 The toxin (1.0 mmol l À1) significantly accelerated the time-to-peak current from 0.62 to 0.52 ms in isoform rNa v 1.2. Higher doses of the toxin (3-10 mmol l À1) additionally decreased time-to-peak current in rNa v 1.4 and rNa v 1.5. 4 A toxin effect on decay of I Na at À20 mV was either absent or marginal even at relatively high doses of CTX3C. 5 The toxin (1 mmol l À1 ) shifted the inactivation potential (V 1/2 of inactivation curve) in the negative direction by 15-18 mV in all isoforms. 6 I Na maxima of the I-V curve (at À20 mV) were suppressed by application of 1.0 mmol l À1 CTX3C to a similar extent (80-85% of the control) in all the three isoforms. Higher doses of CTX3C up to 10 mmol l À1 further suppressed I Na to 61-72% of the control. 7 Recovery from slow inactivation induced by a depolarizing prepulse of intermediate duration (500 ms) was dramatically delayed in the presence of 1.0 mmol l À1 CTX3C, as time constants describing the monoexponential recovery were increased from 3878 to 5887151 ms (n ¼ 5), 5376 to 338785 ms (n ¼ 4), and 2373 to 2327117 ms (n ¼ 3) in rNa v 1.2, rNa v 1.4, and rNa v 1.5, respectively. 8 CTX3C exerted multimodal effects on sodium channels, with simultaneous stimulatory and inhibitory aspects, probably due to the large molecular size (3 nm in length) and lipophilicity of this membrane-spanning toxin.