22Na+ and 36Cl- tracer flux experiments of embryos of Fundulus heteroclitus show that a significant influx and efflux of Na+ and Cl- occur during development. The average influx for Na+ and Cl-, respectively, are 5.0 +/- 0.76 and 1.1 +/- 0.1 nmol . mg-1 . h-1. The average effluxes are 4.9 +/- 0.95 and 3.0 +/- 0.01 nmol . mg-1 . h-1 for Na+ and Cl-, respectively. The electrical potential across the embryonic cavity epithelium of F. heteroclitus and F. bermudae is 50 +/- 1.7 mV and 40 +/- 1.3 mV, respectively (inside positive). Diluting the artificial seawater bathing medium with distilled water decreases the potential across the embryonic cavity of F. bermudae to -11 +/- 5.0 mV at 5% artificial seawater. Because in seawater the potential across the embryonic cavity epithelium is inside positive, the efflux of Cl- is against its electrical potential gradient indicating an active extrusion mechanism for Cl-. The site of the active extrusion mechanism for chloride is located in the yolk sac where chloride cells associated with blood vessels are common.