The effects of isoquinolinesulfonamides, which inhibit protein kinase, on fluid accumulations induced by heat-stable enterotoxin of enterotoxigenic Escherichia coli (STh), 8-bromo-cGMP and 8-bromo-cAMP in suckling mice were studied. Both N-[2-(methylamino)ethyl]-5-isoquinolinesulfonamide (H-8) and N-(2-aminoethyl)-5-isoquinolinesulfonamide (H-9) inhibited the fluid accumulation induced by STh. Fluid accumulation induced by four mouse units of STh (four-fold the minimum effective dose, 10 ng) was completely inhibited by 0.4 mumol of H-8 or H-9. H-8 and H-9 also inhibited fluid accumulation induced by 8-bromo-cGMP. On the other hand, H-8 and H-9 only partially inhibited fluid accumulation in suckling mice induced by 8-bromo-cAMP, probably because their affinities to cAMP-dependent protein kinase were lower than their affinities to cGMP-dependent protein kinase. From these results, it is concluded that the activation of cGMP-dependent protein kinase by increase in cGMP by ST or by 8-bromo-cGMP, and very probably the activation of cAMP-dependent protein kinase by increase in cAMP by cholera enterotoxin and heat-labile enterotoxin of enterotoxigenic E. coli and by 8-bromo-cAMP are necessary steps in signal transduction following increases in concentrations of cGMP and cAMP in intestinal brush border cells.