Intracellular recordings from nucleus gigantocellularis (NGC) neurones were obtained in isolated and perfused whole brains of guinea pigs in vitro. A majority of cells (90%) were characterized by an action potential of short duration (0.3 ms) followed first by a fast and then by a slower afterhyperpolarization (AHP). Their firing pattern was mostly irregular. These cells were shown to have high threshold calcium spikes and plateau potentials. The other cell type represented only 10% of the recorded cells in the NGC. It was characterized by a wider (0.6 ms) action potential, a large single AHP, the presence of a transient rectification presumably due to an A-current and a rather regular resting discharge. Using Lucifer yellow injections in brainstem slices, both cell types were shown to correspond to gigantocellular neurones.