The osmolar effect induced by Pacific ciguatoxin (P-CTX-1) was tested on frog erythrocytes in the search of a biological test to identify active principles contained in plants traditionally used against ciguatera in New-Caledonia. Frog erythrocytes are small elliptic discs [7] whose geometry (length; width) was measured under microscope before and after application of various substances. Variation of the apparent surface area was examined. Erythrocyte surface area was not sensitive to: i) Na+ channels blockade by TTX (0.6 µM); ii) Na+-Ca2+ exchange inhibition by benzamil (1 µM); iii) the absence of Ca2+ in the physiological solution; iv) Ca2+ channels inhibition by Cd2+(1 mM) or verapamil (1 µM). P-CTX-1 (0.1 to 10 nM) increased the surface area. This effect was not blocked by TTX or benzamil. It was suppressed and prevented by Cd2+or verapamil, and did not develop in a Ca2+-free solution. The P-CTX-1 (1 nM)-induced swelling was prevented and suppressed by application of a crude extract isolated from Argusia argentea (10 ng/ml). The data show that erythrocytes swelling induced by PCTX-1 depends on extracellular Ca2+ and on Ca2+ movements. They suggest that P-CTX-1 induced a Ca2+ influx via Ca2+ channels leading to an increase in intracellular Ca2+ concentration which may release Ca2+ from intracellular sites [8]. This, in turn, possibly led to a K+ efflux and to water movement. Data also show that Argusia argentea extract contained some active principles against the swelling effect of P-CTX-1.