This study was to investigate if endogenous pituitary adenylate cyclase-activating polypeptide (PACAP) can be released during direct splanchnic nerve stimulation in vivo, and to determine whether PACAP in the adrenal gland can modulate the medullary response to sympathoadrenal reflex. The output of adrenal catecholamine and PACAP-38-like immunoreactivity (PACAP-38-ir) increased in a frequency-dependent manner following direct splanchnic nerve stimulation (0.2-20 Hz). The both responses were highly reproducible and PACAP-38-ir output closely correlated with catecholamine output. Sodium nitroprusside (SNP; 0.1 mg/kg, iv, bolus) caused a severe hypotension resulting in marked increases in catecholamine secretion. In the presence of local PACAP-27 (125 ng), the maximum catecholamine response to SNP was significantly potentiated in a synergistic manner as compared with that obtained in the group receiving SNP or PACAP-27 alone. The study indicates that endogenous PACAP-38 can be released particularly when the sympathoadrenal system is highly activated and that the local exogenous PACAP-27 enhanced the reflex-induced catecholamine release, suggesting collectively a facilitating role of PACAP as neuromodulator in the sympathoadrenal function in vivo.