The responses of heart rate and blood pressure to noxious radiant heat were studied in seven pentobarbital-anesthetized cats. Afferent activity recorded from the tibial nerve, systemic blood pressure, and heart rate were monitored as skin temperature of the hind footpad was raised to 53 degrees C for 20 s using radiant heat. The averaged tibial afferent nerve activity increased markedly as skin temperature approached 52 degrees C. Within 2-3 s of the onset of increased tibial nerve activity, systolic blood pressure increased an average of 32 mmHg and heart rate increased an average of 16 beats/min in the seven animals that were studied. The results of this study provide evidence for a somatosympathetic reflex that is initiated by cutaneous nociceptors. Under pentobarbital anesthesia, an increase in heart rate and blood pressure appears to be a reliable indicator of nociceptor activation.