Summary The secreting in vitro guinea pig gastric mucosa showed an H+ rate of 3 μE cm-2 hr-1, and an Isc of 57 μa cm-2 hr-1. The latter was accounted for by net N->S Cl- transport and H+ was sensitive to SCN- and N2. Substitution of SO4” for Cl-7 and choline for Na+ inverted the P.D. if H+ was maintained. An explanation for in vitro species differences is proposed.