1. The effects of GABA(B), opioid and alpha2 receptor activation on different subtypes of calcium channels in acutely-dissociated rat locus coeruleus (LC) neurones were investigated using whole-cell patch clamping. 2. Barium currents through calcium channels could be fractionated into four classes: L-type (nimodipine-sensitive), N-type (omega-conotoxin GVIA-sensitive), P/Q-type (omega-agatoxin IVA-sensitive) and R-type (remaining in the presence of all three blockers). The percentage of each was, respectively, 25+/-2, 34+/-1, 28+/-3 and 12+/-1% (mean+/-s.e.mean, n=4). 3. The GABA(B) receptor agonist, baclofen, and the opioid receptor agonist, enkephalin, partially inhibited the total barium current in a concentration-dependent manner with EC50 values of 2 and 0.3 microm , respectively. Maximal inhibition was 17+/-1% (n=38) for baclofen and 30+/-2% (n=20) for enkephalin. The alpha2-adrenoceptor agonist, UK14304 (10 microM), also inhibited barium current in these neurones (28+/-2%, n=11). The agonists did not shift the current-voltage relationship along the voltage axis. 4. Maximal baclofen inhibition of different calcium channel subtypes was 9+/-7% (L-type, n=4), 11+/-8% (N-type, n=4), 26+/-6% (P/Q-type, n=4), and 6+/-5% (R-type, n=5). The corresponding values for enkephalin inhibition were 5+/-9% (L-type), 30+/-11% (N-type), 37+/-9% (P/Q-type), and 17+/-8% (R-type). 5. In the presence of a saturating concentration of enkephalin, baclofen produced additional inhibition of the barium current. In contrast, in the presence of a saturating concentration of enkephalin, UK14304 produced no further inhibition of the barium current. 6. These results indicate that neuromodulation of calcium channels in LC neurones involves a complex pattern of overlapping and distinct second messenger pathways. Regulation of LC neuronal firing activity by the modulation of calcium channels may be important for LC-mediated behaviour such as alertness and vigilance.