1. The actions of the neuropeptide nociceptin on the calcium channel currents (IBa) of acutely dissociated rat periaqueductal grey (PAG) neurons were examined using whole-cell patch clamp techniques. These effects were compared with those of opioid receptor agonists and the GABAB receptor agonist baclofen. 2. Neurons from young adult rats (23 to 56 days old) expressed predominantly omega-conotoxin GVIA (N-type)- and omega-agatoxin IVA (P/Q-type)-sensitive IBa, together with smaller amounts of nimodipine-sensitive current and current resistant to all three blockers. There was proportionately more N-type IBa in neurons from female rats and proportionately more resistant current in neurons from male rats. 3. Nociceptin (EC50, 5 nM) and baclofen (EC50, 0.8 microM) inhibited IBa in all PAG neurons, while the opioid agonist methionine enkephalin (met-enkephalin; 300 nM-10 microM) inhibited IBa in 40 % of neurons. The effects of met-enkephalin were reversed by the mu-opioid antagonist CTAP, and mimicked by the mu-opioid agonist DAMGO (300 nM-3 microM). The delta-opioid agonists DPDPE and deltorphin II, and the kappa-opioid agonist U69593, did not affect IBa in any neuron. The actions of nociceptin were not mimicked or blocked by the opioid antagonist naloxone or the nociceptin analogue [desPhe1]-nociceptin. 4. The effects of nociceptin and baclofen on IBa were blocked by pretreatment of the neurons with pertussis toxin (500 ng ml-1, 8 h). 5. Nociceptin predominantly inhibited the N-type (EC50, 2 nM; maximum inhibition, 50 %) and P/Q-type (EC50, 7 nM; maximum inhibition, 33 %) IBa while having little effect on the L-type and R-type IBa. 6. These results are consistent with the previously described actions of nociceptin, baclofen and micro-opioids in PAG slices, whereby they couple to increases in an inwardly rectifying K+ conductance. These agonists thus have the potential to modulate the function of PAG neurons via a number of different cellular effectors.