In this paper, we show that an intracellular alkalinization of the dorsal ectoderm cells is among the earliest responses to neural induction in Xenopus. Planar explants of the dorsal marginal zone were prepared from embryos that had been microinjected during cleavage stages with the fluorescent pH indicator bis-carboxyethyl-carboxyfluorescein-dextran (BCECF-dextran), and intracellular pH (pHi) was monitored continuously by emission ratio microfluorimetry. During stage 10.5, the dorsal ectoderm cells undergo a sustained intracellular alkalinization of approximately 0.1 pH units in response to neural induction; in the absence of the inductive signal, the pH of the dorsal ectoderm cells decreases slightly. Ectoderm cells within planar explants of the ventral marginal zone show little change in pH during a similar period. This increase in intracellular pH is inhibited by 4, 4'-dihydrodiisothiocyanatostilbene-2, 2'-disulfonate (H2DIDS) or a low Na+/high Cl- medium, treatments that presumably affect anion transport. Under these conditions, expression of the anterior neural-specific homeobox gene engrailed is not detected, while the notochord-specific epitope recognized by the Tor-70 antibody is expressed in the presence of H2DIDS. This characteristic alkalinization is not evoked by pharmacological agents that reportedly alter ectodermal developmental pathways in Xenopus embryos, such as NH4Cl, phorbol esters, or cAMP-dependent protein kinase agonists. Our results suggest that an ionic regulatory event may participate in the regulation of gene expression in response to neural induction.