Intracellular pH (pH_i) is an important modulator of cardiac function. The spatial regulation of pH within the cytoplasm depends, in part, on intracellular H⁺ (\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{H}_{\mathrm{i}}^{+}\) \end{document}) mobility. The apparent diffusion coefficient for \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{H}_{\mathrm{i}}^{+},{\ }D_{\mathrm{H}}^{\mathrm{app}}\) \end{document}, was estimated in single ventricular myocytes isolated from the rat, guinea pig, and rabbit. \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(D_{\mathrm{H}}^{\mathrm{app}}\) \end{document} was derived by best-fitting predictions of a two-dimensional model of H⁺ diffusion to the local rise of intracellular [H⁺], recorded confocally (ratiometric seminaphthorhodafluor fluorescence) downstream from an acid-filled, whole cell patch pipette. Under \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{CO}_{2}{/}\mathrm{HCO}_{3}^{-}\mathrm{-free}\) \end{document} conditions, \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(D_{\mathrm{H}}^{\mathrm{app}}\) \end{document} was similar in all three species (mean values: 8–12.5 × 10^–7 cm²/s) and was over 200-fold lower than that for H⁺ in water. In guinea pig myocytes, \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(D_{\mathrm{H}}^{\mathrm{app}}\) \end{document} was increased 2.5-fold in the presence of \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{CO}_{2}{/}\mathrm{HCO}_{3}^{-}\) \end{document} buffer, in agreement with previous observations in rabbit myocytes. \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{H}_{\mathrm{i}}^{+}\) \end{document} mobility is therefore low in cardiac cells, a feature that may predispose them to the generation of pH_i gradients in response to sarcolemmal acid/base transport or local cytoplasmic acid production. Low \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{H}_{\mathrm{i}}^{+}\) \end{document} mobility most likely results from H⁺ shuttling among cytoplasmic mobile and fixed buffers. This hypothesis was explored by comparing the pH_i dependence of intrinsic, intracellular buffering capacity, measured for all three species, and subdividing buffering into mobile and fixed fractions. The proportion of buffer that is mobile will be the main determinant of \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(D_{\mathrm{H}}^{\mathrm{app}}\) \end{document}. At a given pH_i, this proportion appeared to be similar in all …