We have reevaluated the presumed requirement for an elevated intracellular pH (pHi) in the acceleration of protein synthesis which follows fertilization of eggs of the sea urchin Lytechinus pictus. Zygotes were transferred to sea water at a low pH (6.8) containing a permeant weak acid at times ranging from 5 min to as early as 30 sec postinsemination, to reverse or prevent the rise in pHi that normally ensues upon fertilization. Using the fluorescent pH probe 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF), we show that transfer of zygotes at 1 min or earlier after fertilization essentially blocks the rise in pHi. Under these conditions, both the incorporation of radiolabeled leucine into protein and the assembly of ribosomes into polysomes increase substantially (> 50% of control values). We also assessed leucine incorporation during incubation of eggs and zygotes in sodium-free sea water or sea water containing amiloride, two additional treatments that block the pHi rise. In the presence of amiloride, leucine incorporation increased upon fertilization, whereas little or no increase was observed in sodium-free sea water. We provide evidence that the low rates of leucine incorporation in sodium-free sea water result from the tendency for this experimental condition to lower the pHi to values significantly lower than the pHi in unfertilized eggs. These findings call into doubt the belief that the pHi rise at fertilization is a necessary prerequisite for the acceleration of bulk protein synthesis. These observations support the view that pHi is only one of several signals involved in the turn on of protein synthesis at the time of fertilization of sea urchin eggs.