Lasp1 (LIM and SH3 domain protein1) is a multi-domain actin binding protein that is differentially expressed within epithelial tissues and brain. In the gastric mucosa, Lasp1 is highly expressed in the HCl-secreting parietal cell, where it is prominently localized within the F-actin rich subcellular regions. Histamine-induced elevation of parietal cell [cAMP]i increases Lasp1 phosphorylation, which is correlated with activation of HCl secretion. To determine if Lasp1 is involved in the regulation of HCl secretion in vivo, we generated a murine model with a targeted disruption of the Lasp1 gene. Lasp1-null mice had slightly lower body weights, but developed normally and had no overt phenotypic abnormalities. Basal HCl secretion was unaffected by loss of Lasp1, but histamine stimulation induced a more robust acid secretory response in Lasp1 null mice as compared to wild type littermates. A similar effect of histamine was observed in isolated gastric glands based on measurements of accumulation the weak base, [¹⁴C]-aminopyrine (AP). In addition, inhibition of the acid secretory response to histamine by H2 receptor blockade with ranitidine proceeded more slowly in glands from Lasp1 null mice. These findings support the conclusion that Lasp1 is involved in the regulation of parietal HCl secretion. We speculate that cAMP-dependent phosphorylation of Lasp1 alters interactions with F-actin and/or endocytic proteins that interact with Lasp1, thereby regulating the trafficking/activation of the H⁺, K⁺-ATPase (proton pump).