The rapid transductional sequences initiated by interferon-γ (IFN-γ) on binding to its receptor regulate functional and genomic responses in many cells but are not well defined. Induction of macrophage activation is an example of such functional and genomic changes in response to IFN-γ. Addition of IFN-γ to murine macrophages, at activating concentrations, produced rapid (within 60 seconds) alkalinization of the cytosol and a concomitant, rapid influx of 22Na+. Amiloride inhibited the ion fluxes and the accumulation of specific messenger RNA for two genes induced by IFN-γ (the early gene JE and the β chain of the class II major histocompatibility complex gene I-A). The data indicate that IFN-γ initiates rapid exchange of Na+ and H+ by means of the Na+/H+ antiporter and that these amiloride-sensitive ion fluxes are important to some of the genomic effects of IFN-γ.