Endocarditis, involving the left atrioventricular valve, was produced in 6 of 10 pigs as early as 18 h after a single intravenous inoculation of a group L streptococcus. Macroscopic vegetations were not detected in pigs killed at 12 or 15 h after inoculation. The number of streptococci in the blood during the course of infection was monitored at 3-h intervals. No significant increase in the number of bacteria coincident with the early stages of the development of lesions was found. It is suggested that non-bacterial thrombotic endocarditis is not a prerequisite for the development of bacterial endocarditis. The development of heart valve vegetations is possibly initiated immediately after the intravenous inoculation of streptococci.