Identifying brain systems that regulate or modulate autonomic nervous system functions may identify pathways through which psychosocial factors can influence health and disease. Reduced high-frequency heart rate variability (HF-HRV) characterizes anxiety disordered patients and is predictive of adverse myocardial events. Sex differences in the prevalence of anxiety disorders and cardiac diseases implicate the possibility of sex specific neural regulation of HF-HRV. We investigated the correlation between HF-HRV and regional cerebral blood flow (rCBF) in 28 subjects (15 women) with social phobia undergoing a stressful public speaking task. Regional CBF was measured with [(15)O] water positron emission tomography. Stress induced rCBF correlated positively with HF-HRV in the right supra genual anterior cingulate cortex Brodmann\'s area (BA) 32, the right head of the caudate nucleus and bilaterally in the medial prefrontal cortex (BA10), extending into the dorsolateral prefrontal cortex (BA46) in the left hemisphere. Men showed larger positive co-variation in the caudate than women. These findings underscore the importance of the emotional division of the anterior cingulate cortex, the prefrontal cortex and the striatum in cardiovagal activity. The study replicates and extends results from published functional neuroimaging studies on cardioregulatory or modulatory areas in healthy subjects to men and women with social phobia. Moreover, caudate functions, possibly related to dopaminergic neurotransmission, have sexually dimorphic effects on vagal modulation of the heart.