We have examined the effect of caerulein on intestinal fluid transport in vivo. Rat jejunal, ileal, and colonic segments were perfused with a physiologic buffer for a 60-min control period, followed by a 60-min period after caerulein, prostaglandin E2 (PGE2), or saline (0.9%, w/v) were given by intramuscular (i.m) injection. At a dose of 5 micrograms/kg caerulein had no effect on ileal fluid transport. Lower doses of caerulein (300 ng and 1 microgram/kg) also had no effect. PGE2 significantly (P less than 0.05) inhibited net ileal absorption by 94 +/- 24% from pre-dose levels. Caerulein (300 ng/kg) had no effect (P greater than 0.05) on jejunal or colonic fluid transport. Intestinal fluid accumulation assessed by the 'enteropooling' assay did not increase after 30 ng or 1 microgram/kg of caerulein, whereas in animals given PGE2 of (5 mg/kg) the fluid accumulation more than doubled (P less than 0.001): control 1.59 +/- 0.15 ml; cerulein (1 microgram/kg) 1.36 +/- 0.20 ml; and PGE2 4.7 +/- 0.50 ml. Serum levels of caerulein (after a 1 microgram/kg dose), measured by radioimmunoassay, were elevated up to 30 min after i.m. injection. The data indicate that caerulein has no direct effect on rat small or large intestinal fluid transport.