CSF and plasma GABA was measured in patients with bipolar affective illness and matched controls. There was no correlation between CSF and plasma GABA in 10 pairs of simultaneously obtained samples. Plasma GABA was significantly lower in a group of 10 euthymic medication-free bipolar patients compared to 41 normal volunteers (P <.02). This finding was replicated in a second study involving 6 patients and 10 normal volunteers. Twin studies showed a high correlation between members of 11 monozygotic twin pairs (P <.008). Diurnal variation was minimal, but bimonthly sampling over one year revealed somewhat higher values in the late summer compared to late winter.