Obesity is known as a risk factor in stress-related cardiovascular pathology in man. The length of obesity can be an important interacting variable. Therefore, cardiac and behavioral responses to emotional stress were studied in 1-year-old, genetically obese (fa/fa) and lean(Fa/-) male Zucker rats, a frequently used animal of genetic obesity. An early bradycardic response to emotional stress evoked by stimuli associated with brief previous inescapable foot shock, as observed in lean rats, was absent in the fatty Zuckers. This difference was not due to a learning deficit: obese and lean Zuckers showed the same degree of conditioned behavioral responses to the emotional stress. Furthermore, the magnitude of the novelty induced behavioral arousal was also comparable. As far as the regulation of body temperature is concerned, the fa/fa rats displayed a diminished increment in rectal temperature in response to the emotional stress. In conclusion, the results showed an impairment of phasic change in the parasympathetic drive of heart to emotional stress in the long-term obese animals. The diminished activation of heat production points to a blunted response of certain branches of the sympathetic nervous system to emotional stress. The findings favor the hypothesis that dysfunction of cardiac vagal drive in relation to stress is of pathologic importance in long-term obesity.