Forty-eight heterotopic heart transplants (33 homografts and 15 isografts) were performed in inbred rats. Levels of kallikrein, prekallikrein, and kallikrein inhibitor were determined in coronary sinus venous blood of grafted hearts at various intervals postgrafting. Homograft observations were assessed against those in nonrejecting isografts. Significant increases in homograft prekallikrein level on day 4 were followed by significant increases in homograft kallikrein activity on day 6. Kallikrein inhibitor levels in homografts were consistently increased in relation to isograft levels. Kallikrein and inhibitor activity showed an inverse relationship in their respective changes. High levels of kallikrein and prekallikrein occurred in homografts ten days posttransplantation. Immunological rejecting homografts showed meaningful changes in the activity of kinin-forming substances known to participate in inflammation.