Neurons in the medullary reticular formation were tested for responses to mechanical stimuli applied to the heart in cats anesthetized with chloralose and paralyzed with pancuronium. In most experiments baroreceptors were denervated. Aortic occlusion excited 15 neurons (19%) and decreased the mean discharge rates of five neurons (6%). Discrete probing of the heart elicited one to three spikes from 18 of 27 neurons tested. Thirteen of these cells had defined cardiac receptive fields; fields were large, often encompassing most of the left ventricle. Of 12 neurons tested for responses during fibrillation, 8 were excited, 2 were inhibited, and 2 were unaffected. Neurons often exhibited different sensitivities to these mechanical stimuli, as well as to ischemia produced during coronary arterial occlusion. Neurons were more likely to respond to stimuli that affected a large mass of myocardium. In addition to cardiac input, 98% of neurons in this study also received input from at least one additional sensory modality, and 39 cells were excited by somatic, auditory, and visual stimuli. Results indicate that medullary reticular neurons are responsive to mechanical events in the heart as well as to myocardial ischemia and respond to other sensory modalities.