To study the pathophysiology of diarrhoea in coccidial infections, Na+ and Cl- fluxes (J(Na), J(Cl)), short circuit current (I-sc) and tissue conductance (g(t)) were determined in stripped gut epithelia of Eimeria separata infected rats employing the Ussing chamber technique. E. separata invades enterocytes of the caecum and proximal colon. Na+ absorption was generally reduced in infected tissues, Cl- absorption only in the caecum. I-sc values were increased in the caecum and reduced in the proximal colon. Tissue conductance was not affected. Values tended to normal with time after infection. Theophylline caused markedly increased I-sc and g(t) values in the caecum epithelia of infected rats. In the epithelia of the distal colon, i.e. the non-infested part of the large intestine, g(t) values remained unaffected but I-sc was fourfold increased. This I-sc increase was strongly sensitive to amiloride, suggesting a compensatory activation of Na+ channels in the distal colon of infected rats. Accordingly, serum levels of aldosterone, which activates Na+ channels in the distal colon, were increased to eightfold levels in infected animals. Thus compensatory Na+ absorption was under endocrine control.