Interspecific interactions between Nippostrongylus brasiliensis and Eimeria nieschulzi were studied by measuring parasite fecundity and patency in rats concurrently infected with both species. Nine groups of five rats each were inoculated with either 4 x 10(3) N. brasiliensis larvae or 2.6 x 10(5) E. nieschulzi sporulated oocysts or both. Groups 1 and 2 served as singly-infected controls. Group 3 rats were infected simultaneously with both parasites. Rats in Groups 4 to 8 were infected first with N. brasiliensis and later with E. nieschulzi on days 3, 4, 8, 9, and 14 PI. Rats in Group 9 were infected with E. nieschulzi and then with N. brasiliensis 2 days later. Patency of N. brasiliensis infections was extended significantly (P less than or equal to 0.05) beyond singly-infected controls in all groups with concurrent infections except Group 3, whereas ova production was significantly greater than that in singly-infected controls only when both parasites were administered simultaneously (Group 3). Patency of E. nieschulzi was significantly shorter in Group 7 rats and longer in Group 9 rats compared to that of singly-infected controls. Total oocyst production was unaltered except for Groups 5 and 7 in which there was a significant reduction. These results demonstrate that the endogenous stages of E. nieschulzi increased N. brasiliensis longevity.