The effect of local graft irradiation on the function of heterotopically transplanted heart has been studied in rats. Lewis strain rats received abdominal heart transplants from donors of the Buffalo strain. After transplantation, recipients were randomly assigned to one of two groups group I received six doses of 150 R each, delivered only to the graft area, and group II remained untreated. The mean, duration of function differed significantly for untreated grafts (9.9 days) and irradiated grafts (14.7 days). Lymphocytic infiltration was delayed in irradiated hearts. © 1969 by The Williams and Wilkins Co.