This study was conducted to investigate whether there are differences in the autonomic nervous system function of chickens from lines selected for high (HWS) or low body weight (LWS). The cardiovascular response to various pharmacological agents was used as an indicator of autonomic nervous system response. Ten individuals from each line and sex were used in the study. Catheters were introduced into the left brachial artery and vein and connected to a MP100-BIOPAC system to record blood pressure and heart rate (HR). Chickens were injected with phenylephrine, atropine, propranolol, and tetraethylammonium chloride (TEAC). The LWS birds exhibited a greater increase in mean arterial blood pressure (MABP) and a lesser increase in HR than the HWS birds following atropine. The response to atropine showed a line and sex interaction in which male birds had a greater increase in HR than females and LWS females had a lower increase in HR than the HWS females. Injection of phenylephrine following pretreatment with atropine caused a baroreceptor reflex in which males showed a greater decrease in HR than females. In response to the beta-adrenergic receptor blocker propranolol, females displayed a greater decrease in MABP than males and LWS birds had a greater decrease in HR than HWS birds. In response to the autonomic ganglionic blocker TEAC, MABP and HR decreased equally in both lines. The percentage of adrenal and sympathetic impact on regulation of HR showed that LWS females required greater adrenal activity than those from the other subclasses. Although changes in HR and MABP ratios in response to phenylephrine were different between lines, these responses were not different when phenylephrine was given following atropine. This pattern of response suggested that HWS birds had greater parasympathetic nervous system activity in order to maintain cardiovascular function. These results demonstrate that selection for HWS or LWS has resulted in greater parasympathetic and sympathetic nervous system tone in birds from the HWS and LWS birds, respectively, and suggest that differences between the lines could be at the level of the chromaffin tissue in the adrenal gland.