Potential therapeutic benefits of dietary n-3 fatty acids are presently evaluated in arteriosclerotic and chronic inflammatory diseases. Because some other long-chain fatty acids inhibit intestinal fluid absorption, the effects of docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) on fluid and electrolyte transport were tested in rat colon and jejunum. In ligated colon loops in vivo, DHA (10(-5)-10(-3) M luminally) diminished net sodium and fluid absorption, and stimulated net potassium secretion. Similar but smaller effects were produced by EPA. In the jejunum, transport parameters were not influenced by DHA and EPA. In isolated distal colonic mucosa in vitro, DHA (10(-4)-10(-3) M on the mucosal side) reduced the electrical resistance, potential difference, short-circuit current and the net absorption of sodium and chloride due to increased unidirectional serosal to mucosal fluxes. These changes were prevented when calcium was replaced by strontium in the bathing solution. The data suggest that n-3 fatty acids increase colonic conductance indirectly by binding luminal calcium.