The sequence of pathological changes in both B. argentina and B. bigemina infections of splenectomised calves are described. Renal damage preceded hepatic damage by one to two days. BUN levels increased 2-4 times; SGOT increased 4 times. Unconjugated bilirubin levels increased from {reversed tilde equals} 0.30 mg% to a peak 4.12 mg%±1.90 mg% in one group and up to 8-10 times in others. Conjugated bilirubin levels fell terminally. BSP fractional clearance levels fell in severe B. argentina cases but remained relatively unaltered in B. bigemina infections. A biphasic BSP fractional clearance curve was present in both normal and infected calves. Serum and urine sodium concentrations remained static. Serum potassium concentrations fell in some B. bigemina and B. argentina infected calves. Urine potassium levels rose in all groups. Tubulonephrosis and degeneration of the convoluted and collecting tubules was present in severe infections. Vascular congestion was frequently observed, but vascular stasis only occurred in severe B. argentina cases. Glomerular shrinkage was usual in moderate and severe cases. Severe hepatic centrilobular and midzonal degeneration and necrosis was common in all severe cases. Vascular congestion accompanied this, but stasis was present only in some B. argentina infections.