GABA(B) receptors have been implicated in the GABAergic modulation of catecholaminergic and serotonergic pathways in the central nervous system. The GABA(B) receptor may require two subunits, GABA(B)R1 and GABA(B)R2, for functional activity. Using dual immunofluorescent labelling on adjacent cryostat sections, we investigated the presence of immunoreactivity for the GABA(B)R1 and GABA(B)R2 subunits in brainstem catecholamine (tyrosine hydroxylase-immunoreactive) and serotonin (tryptophan hydroxylase-immunoreactive) neurons. All neurons (>98%) examined in catecholamine groups A1, A2, A5, A6, C1, and serotonin groups B1-3 and B6-8 were immunoreactive for the GABA(B)R1 subunit. All A5 and A6 neurons (>97%) and at least 86% of A1, A2, C1, B2, B3, B7 and B8 neurons examined were GABA(B)R2-immunoreactive. The proportion of neurons with immunoreactivity for the GABA(B)R2 subunit varied between 0% and 99% for B1 neurons, and between 35% and 93% for B6 neurons. Statistical analysis showed that similar proportions of sampled neurons were immunoreactive for GABA(B)R1 and GABA(B)R2 in the A1, A5, A6, C1, B2 and B7 cell groups, whereas a smaller proportion of A2, B1, B3, B6 and B8 neurons were GABA(B)R2-immunoreactive than GABA(B)R1-immunoreactive. In general, our results suggest that GABA(B)R1 and GABA(B)R2 co-exist in the great majority of brainstem catecholamine and serotonin neurons. In the neurons that lack GABA(B)R2, the GABA(B)R1 subunit may act alone or with another protein.