Intracellular recordings were made from neurones contained in the locus coeruleus and mesencephalic nucleus of the trigeminal nerve (MNV), in tissue slices cut from guinea-pig pons and maintained in vitro. Locus coeruleus neurones were of -52.7 +/- 2.7 mV resting membrane potential; had an input resistance of 58.0 +/- 7.6 M omega and a membrane time constant of 7.3 +/- 1.0 ms. These neurones fired action potentials in response to depolarizing current pulses. Depolarizing synaptic potentials (DSPs) were recorded in locus coeruleus neurones in response to focal stimulation of the surface of the slice. MNV neurones were of -51.9 +/- 3.6 mV resting membrane potential; had an input resistance of 15.0 +/- 1.8 M omega and a membrane time constant of 1.35 +/- 0.16 ms. These neurones were also excitable but differed from locus coeruleus neurones in that they showed accommodation to depolarizing current pulses and time-dependent anomalous rectification with hyperpolarizing current pulses. In MNV neurones focal stimulation did not give rise to DSPs. Intracellular injection of Lucifer yellow revealed that the cell bodies of locus coeruleus neurones were small and multipolar whereas MNV neurones had larger, monopolar cell bodies.