The aim of this study was to characterize the dynamic changes in uterine fluid composition and acid-base status during shell calcification in the chicken. Uterine eggs at timed intervals were manually aborted and the accompanying fluid collected and analyzed for composition of osmolytes, enzymes, and acid-base parameters. Blood samples were analyzed for comparison. No considerable change in blood gases took place in relation to residence time of the calcifying egg in the uterus. A significant acidosis occurred at latter stages. Only minor changes were revealed in plasma osmotic and biochemical composition throughout egg calcification. In contrast, major changes were revealed in uterine fluid composition and acid-base status during calcification. The most prominent phenomenon was the sharp increase in CO2 partial pressure, from 82.2 Torr at 0 h to 132.8 Torr at 10 h. As bicarbonate concentration remained almost stable, fluid pH dropped from 7.412 to 7.250 within this stage. Uterine fluid sodium and chloride concentrations and osmolality dropped significantly in the course of calcification, whereas potassium concentration significantly increased. A sharp increase in glucose, calcium, and magnesium concentrations was measured in the early stages of calcification. These findings are discussed in relation to existing models for transport mechanisms of the uterine epithelial tissue. The comprehensive picture that emerges from the present study should enable future application in establishing a self-contained culturing system in vitro for studies of embryonic development.