Methylxanthines increase central noradrenergic turnover by an unknown mechanism. Isobutyl methylxanthine (IBMX, 0.5-16 mg/kg i.v.) was found in this study to increase firing rats of noradrenergic neurons in the locus coeruleus, supporting increased impulse flow as one possible mechanism for increased noradrenergic turnover. Naloxone (1 mg/kg i.p.) pretreatment potentiated the effects of IBMX, while clonidine (0.01 mg/kg i.v.) reversed increases in activity. Some of the behavioral effects of methylxanthines are consistent with increased noradrenergic activity.