We have previously shown that stimulation of acid secretion in parietal cells causes rapid initial cell shrinkage, followed by Na⁺/H⁺-exchange-mediated regulatory volume increase (RVI). The factors leading to the initial cell shrinkage are unknown. We therefore monitored volume changes in cultured rabbit parietal cells by confocal measurement of the cytoplasmic calcein concentration. While blocking the presumably apically located K⁺ channel KCNQ1 with chromanol 293b reduced both the forskolin- and carbachol-induced cell shrinkage, inhibition of Ca²⁺ sensitive K⁺ channels with charybdotoxin (ChTX) strongly inhibited the cell volume decrease after carbachol, but not after forskolin stimulation. The cell shrinkage induced by both secretagogues was partially inhibited by blocking H⁺/K⁺-ATPase with SCH28080, and completely absent after incubation with the Cl^- channel blocker NPPB, which inhibits parietal cell anion conductances involved in acid secretion. The subsequent RVI was strongly inhibited with the NHE1-specific concentration of HOE642, and completely by 500µM DMA which also inhibits NHE4. None of the above substances induced volume changes under baseline conditions. Our results indicate that cell volume decrease associated with acid secretion is dependent on the activation of K⁺ and Cl^- channels by the respective secretagogues. K⁺, Cl^- and water secretion into the secretory canaliculi is thus one likely mechanism of stimulation-associated cell shrinkage in cultured parietal cells. The observed RVI is predominantly mediated by NHE1.