The addition of BaCl2 to the serosal solution, at concentrations from 0 X 25 mM to 10 mM, caused increases in potential difference, short-circuit current and resistance across stripped sheets of rat mid-intestine, although mucosal application had little effect. The response to BaCl2 was significantly greater than that obtained with the same concentration of CaCl2. An increase in net Cl- secretion entirely accounted for the rise in short-circuit current induced by BaCl2. BaCl2 inhibited net fluid uptake by everted sacs. It also enhanced the accumulation of fluid by intestinal loops in vivo and this was associated with an increased potential difference. The response to BaCl2 in vitro was not reduced in the absence of serosal Ca2+ ions. The effect of BaCl2 was abolished by trifluoperazine and also by TMB-8 (8-(N,N-diethylamino)-octyl-3,4,5-trimethoxybenzoate hydrochloride). BaCl2 did not alter cyclic AMP production by isolated enterocytes. It is concluded that BaCl2 induces intestinal secretion by releasing Ca2+ from intracellular stores which then combines with calmodulin to stimulate the secretory process.