Net intestinal fluid movement was measured in immunized and non-immunized rats infected with the enteric stages of the nematode, Trichinella spiralis. Animals were studied 30 min, 5 days and 30 days after receiving infective larvae. Net water movement across the mucosal surface of the gut was measured in vivo by perfusing a cannulated segment (approximately 30 cm) of proximal small intestine with an isotonic solution containing a nonabsorbable marker, 14C polyethylene glycol, at a rate of 0.5 ml/min. Uninfected rats showed net absorption of water, 25 microliter/h per cm of intestine. This response was unaltered when rats were infected (7 x 10(3) larvae/rat) and examined 30 minutes later. Five days after primary infection net tissue-to-lumen fluid movement, ie. net secretion, occurred at a rate of 45 microliter/h per cm. When rats were studied 30 days after primary infection, net absorption equivalent to the preinfection level was observed once again. Previously infected (immunized) rats, when challenged (secondary infection, 7 x 10(3) larvae/rat) and examined within 30 min showed a significant decrease in net absorption rate as compared with non-immunized rats. Absorption returned to the preinfection level at both 5 and 30 days post challenge. Results support the conclusion that T. spiralis caused a decrease in net lumen-to-tissue fluid movement during primary and secondary infections. The response was initiated faster in previously infected hosts. The rapid induction of net fluid movement in the direction of secretion upon secondary contact with the parasite is associated temporarily with prevention of worm establishment.