Noradrenergic and GABA systems may be involved in mania, but there is little information about relationships between the function of these systems and response to specific antimanic treatments. We investigated relationships between indices of catecholamine or GABA system function, pretreatment mania severity and antimanic response to divalproex, lithium, or placebo. Plasma GABA and urinary excretion of catecholamine metabolites were measured before randomization to lithium, divalproex or placebo in patients hospitalized for manic episodes. Severity of mania was evaluated using the Manic Syndrome, Behavior and Ideation and Mania Rating Scale scores from the SADS-C. Multiple regression analysis showed that pretreatment plasma GABA was related to severity of manic symptoms. This relationship seemed stronger in women. Multiple regression analysis showed that pretreatment levels of urinary MHPG correlated with improvement in manic syndrome scores. These data suggest that GABA and norepinephrine may be related to different aspects of the manic state and to its pharmacologic sensitivity.