Challenge infections with 10³, 5 × 10⁴, 10⁵, or 5 × 10⁵ sporulated Eimeria praecox oocysts caused moderate but significant weight gain reduction at all infective doses. Substantial reduction in plasma carotenoids and moderate but significant increases in plasma were observed only at the two higher doses when measured at day 6 postchallenge (PC). Daily monitoring of chickens after challenge with 5 × 10⁴ oocysts revealed an inflammatory response in the duodenum and jejunum beginning at day 1 PC that was associated with a significant increase in levels of plasma , which peaked at day 4 PC. A moderate, uniform hyperplasia of the small intestine and significant depression of plasma carotenoids were observed on days 4–6 PC. Plasma decreased to control levels by day 6 PC. All infections were accompanied by production of a mucoid exudate in the duodenum and jejunum, which became thick and opaque by 4 days PC and tended to obscure mildly inflamed areas. These observations indicate that the acute host response to primary infection with E. praecox is both different from and occurs earlier than the response to experimental infections with other Eimeria spp., such as Eimeria acervulina, Eimeria maxima, or Eimeria tenella. These factors need to be considered in observations of pathology arising from co-infections of E. praecox with other Eimeria species, especially in drug sensitivity testing of Eimeria oocysts recovered from litter and in the evaluation of live oocyst vaccines.