The apparent digestibilities (AD) of dry matter (DM), organic matter (OM), crude ash (CA), crude fiber (CFi), crude fat (CFa) crude protein (CP) and nitrogen-free extracts (NFE) and the nitrogen balance were investigated during experimental Eimeria bovis coccidiosis in calves. noninfected pair-fed controls and controls fed on a normal plan of nutrition were included in the study to allow differentiation between the effects of infection and of changes in feed intake. Primary infection with 5 x 10(4) oocysts (n = 4, group A) caused mild diarrhea and calves infected primarily with 1 X 10(5) oocysts (n = 5, group B) suffered from mild (three calves) to severe hemorrhagic (two calves) diarrhea. No clinical disease was seen after reinfection of the group A calves with 1 X 10(5) oocysts. The primary infection with 5 X 10(4) oocysts or reinfection with twice the primary inoculum did not affect AD of nutrients or the overall nitrogen balance (RT). AD of DM, NFE or OM were higher in group B during patency and in the pair-fed group C calves (n = 5) than in the reinfected but healthy group A calves. AD of CFi of the group B calves even exceeded the values of the pair-fed controls. The two calves of group B that suffered from hemorrhagic diarrhea and anorexia had low values of AD of CP during the acute phase of the disease and the plasma nitrogen levels were reduced in this group. Severe clinical coccidiosis transiently reduced the nitrogen balance. It is concluded that the transient increase of AD of nutrients, especially of CFi, during clinical coccidiosis reflect hypomotility and that anorexia and intestinal leakage impair the nitrogen balance and cause weight depression.