We questioned if the optically transparent noncellular chorion, or egg envelope, which encapsulates the entire medaka fish (Oryzias latipes) embryo might in some way constitute a permeability barrier to high concentrations of the diuretic called amiloride. More specifically, we questioned if removal of cations from the exogenous environment of the medaka embryo might make the chorion more permeable to amiloride and thereby make the fish embryos more sensitive to the inhibitory and lethal effects of this drug. To test this question, chorion-encapsulated medaka embryos were exposed to: deionized-distilled water, to Yamamoto-Ringer's (Y-R) solution, to Yamamoto-Ringer's containing choline chloride as a substitute for NaC1, and to isotonic NaCl solution in the presence of and in the absence of amiloride. Briefly, the prediction that the medaka embryos would be most sensitive to amiloride's inhibitory effects in distilled water was confirmed. Further studies showed that the presence of Naf or of Ca2+ alone in the culture solution gave partial protection against the lethal effects of the amiloride. Electron probe Xray microanalysis studies indicated that addition of Ca2+and other cations to the culture solution caused the concentrations of cations to increase in the chorion, and that increase was correlated to a visible decrease in the permeability of the chorion to the amiloride. This decreased permeability of the chorion apparently protected the embryo from the amiloride. The decreased permeability of the chorion to amiloride, which occurred in the presence of the cations present in Y-R solution, was found to be reversible once the cations were washed from the chorion. Key words medaka, chorion, N a f , Ca2+permeability, x-ray microanalysis, Oryzias latipes, egg envelope 'All eggs were collected and placed in amiloride at the four-cell stage (stage 5). 'Stage 16 is late gastrula where % of the yolk sphere was covered by the blastoderm. 'Neither P nor M 2 + were present in detectable levels in the chorion. 'The number of measurements ranged from 5 to 10. 3Values in each column which share a common line are not significantly different, whereas all values which do not share a common line are significantly different at p < 0.001,