The techniques for the measurement of ion influxes and ion effluxes with specially designed Ussing chambers have been described earlier (41-43). Briefly, the calvarium of an embryonic chick, after dissection, was clamped between two chambers of 3-to 4-ml capacity. Ef&x through the outer “periosteal” or inner membrane “endosteal” was measured separately by the appearance of lactate in the medium. Two modifications have been employed in the present work. First, in addition to embryonic chick calvaria, specimens were taken from 2-to 3-day-old, 6-to 10-g rat pups. Two strains of rats were studied, Sprague-Dawley, obtained from Holtzman Co., or Wistar, bred