The possible involvement of basal telencephalic structures in visually conditioned heart rate change (established by pairing light and foot-shock) was studied in 156 pigeons by evaluating conditioning performance following lesions of the septum or lobus parolfactorius. Extensive destruction of the septal complex had no effect on either the orienting response or the development of the conditioned response. Lesions of the lobus parolfactorius did not affect the orienting response or overall conditioned response levels, but it did slightly prolong the latency of the conditioned heart rate change. It is concluded that the septum, despite its being cardioactive, is not involved in conditioned heart rate change and that the lobus parolfactorius participates minimally. Thus, of the principal limbic structures of the avian telencephalon, only the amygdalar homologue appears critical in this defensive conditioning task [3].