Individual differences in cardiovascular regulation result from a dynamic interaction between genetically programmed developmental processes and extrinsic (environmental) conditions. The alterations in autonomic functioning previously identified among pups of the spontaneously hypertensive rat (SHR) strain suggest a genetic influence on autonomic development. In the present studies, an early stimulation procedure previously shown to reduce the behavioral and endocrine responsiveness of rodents in adulthood was demonstrated to alter both neural control of heart rate and regional brain catecholamine levels during the preweaning period. Pups from litters of SHR and Wistar-Kyoto (WKY) rats were handled daily during the first postnatal week. Selective pharmacological blockade with atenolol and atropine methylnitrate was used to identify autonomic controls of heart rate at 16 and 28 days of age. Combined blockade was used to estimate intrinsic heart rate. In both SHR and WKY, early handling was associated with elevated basal heart rate, elevated intrinsic heart rate, and decreased sympathetic tone. Parasympathetic influence on heart rate was elevated by early handling in WKY and decreased by handling in SHR. In addition, an increase in dorsal cortical norepinephrine concentration was produced by early handling in both strains. These data suggest that early environmental conditions can interact with the genetic predisposition for elevated blood pressure and may play a potentially important role in organizing the neural controls of cardiovascular regulation.