Jugular blood samples were taken at regular intervals from 31 ketosis-prone cows from 2 wk prepartum to 7 wk postpartum. Eleven cows exhibited elevated blood ketones and depressed blood glucose indicative of subclinical ketosis. There were no significant differences between means of normal and subclinically-ketotic cows in serum insulin or blood metabolites prior to calving. However, in early lactation, those cows which developed ketosis showed depressed serum insulin, blood glucose, and plasma triglycerides with elevated ketones, acetate in blood, and free fatty acids and cholesterol in plasma. Milk production was also lower in ketotic cows. Correlations within cow between serum insulin and glucose, total ketones, acetate of blood and free fatty acids, triglycerides, and cholesterol in plasma were .014,--.307, .080,--.421, .413, and -.002 for normal cows and .348, -.425, -.324, -.317, .298, and -.131 for subclinically-ketotic cows. It is suggested that low insulin during ketosis is a reflection of depressed blood glucose and, consequently, adipose lipolysis and hepatic ketogenesis are accentuated while acetate utilization and hepatic triglyceride release are depressed.