A radioenzymatic-paper chromatographic method for a simultaneous assay of catecholamines was used to study the effect of cocaine on the release of endogenous catecholamines from the sympathoadrenal medullary system into the blood of unanesthetized rats. Twenty-four hours after arterial cannulation, the "basal" levels of norepinephrine (NE) and epinephrine (EPI) in blood obtained through the catheter from conscious, undisturbed rats were 0.48 +/- 0.06 and 0.36 +/- 0.06 ng/ml, respectively. Administration, via the arterial catheter, of cocaine (0.4-10 mg/kg) produced dose-related increases in NE (0.59 +/- 0.03 to 1.58 +/- 0.34 ng/ml) and EPI (1.15 +/- 0.16 to 6.67 +/- 0.46 ng/ml). Inhibition of catechol O-methyltransferase by tropolone (40 mg/kg) enhanced by 5- to 10-fold the maximal response to cocaine without altering significantly the basal plasma levels of EPI or NE. Bilateral splanchnic denervation reduced the cocaine-tropolone-induced release of EPI and NE by 75 and 50%, respectively. Desipramine (10 mg/kg) failed to alter significantly plasma levels of NE or EPI, even after tropolone. Thus, the increment in plasma levels of NE and EPI in conscious rats given cocaine is mainly the result of a centrally mediated adrenal medullary discharge of catecholamines, rather than inhibition of catecholamine uptake.