1. The intracellular pH in primitive red cells from 4 day chick embryos was measured with the digitonin null-point method and the fluorescent indicator SNARF-1. At physiological pHe of 8.0 red cell pH is 7.39 at day 4. 2. The calculated proton equilibrium potential of -38 mV is in good agreement with previous measurements of Em (Engelke et al., 1988) and supports the conclusion that the Em is dominated by a proton conductance. 3. The sodium-proton exchanger is present in primitive red cells but quiescent under physiological conditions. 4. The results indicate that the bicarbonate-chloride exchange via Band 3 protein is impaired.