Six 34- to 42-day-old lambs raised in coccidia-free conditions were inoculated with 70,000 sporocysts derived from sheep heart with microscopic sarcocysts. Fever and mild anorexia occurred between 25 and 33 days after inoculation. A transient anaemia was most marked 32 days after inoculation. Lambs were killed and examined 14, 25, 33, 42, 60 and 81 days after inoculation. Gross lesions were absent. First and second generation meronts were present in endothelial cells at 25 and 33 days after inoculation. Meronts were most numerous in kidney glomeruli. Developing sarcocysts were rare at 42 days after inoculation. Sarcocysts with a primary cyst wall 2 to 3 micron thick composed of palisade projections were common at 60 and 81 days after inoculation in striated muscle and brain. Mild to severe striated muscle myositis and non-suppurative encephalitis or encephalomyelitis with glial nodules were observed 25 to 81 days after inoculation. Sarcocyst frequency varied considerably; it was highest in myocardium, M vastus intermedius, M vastus medialis, M extensor carpi radialis and tongue muscle and was lowest in M masseter.