Plasma lipid, apolipoprotein (apo) B and apo A-I concentrations were measured in fetal calves in the age range of 4-9 months. The results were compared with those obtained from preruminant calves and cows. The studied lipids and apolipoproteins underwent quantitative underwent quantitative changes during development. Fetal calves, as compared to preruminant calves and cows, had a particularly low apo A-I concentration and a high apo B/apo A-I ratio. Unlike adult animals which have two forms of apo B, fetal calves had only high molecular weight apo B (apo Bh). During fetal development triglyceride, cholesterol and apo B concentrations decreased, but apo A-I concentrations remained constant.