Persistent inward currents (PICs), carried by non-inactivating voltage-sensitive Ca⁺⁺ channels, are considered by many as necessary to produce a strong discharge in motoneurones, by amplification of synaptic excitation or by evoking plateau potentials (Powers and Binder, 2001). We have used intracellular recording and the application of graded depolarizing currents (Hultborn et al., 2003) to investigate the roles of PICs in producing respiratory drive potentials and respiratory discharges in motoneurones innervating a variety of muscles. In cats decerebrated under isofluorane anaesthesia (Hultborn et al. 2003), recordings were made under neuromuscular blockade (pancuronium bromide 0.6 mg.hr^-1) and artificial ventilation, including added CO₂. A strong respiratory drive was evoked, monitored as inspiratory discharges (phrenic or T6 external intercostal nerve). Intracellular recordings were made from inspiratory motoneurones (C5 phrenic or T8 external or internal intercostal nerve), expiratory motoneurones (T8 internal intercostal nerve), or from hindlimb motoneurones (L7/S1). Intracellular electrodes contained K⁺ acetate or, for studying drive potentials without firing, K⁺ acetate with QX314. Discontinuous current clamp (typical sampling 3kHz) was used to monitor membrane potential during the application of currents through the recording electrode. In some inspiratory motoneurones, graded depolarization produced amplification of the excitatory phase of the drive potentials by a factor of about 2, but with only a minimal appearance of any plateau-like behaviour. In expiratory motoneurones graded amplification was not detected, but some of these motoneurones demonstrated clear plateau potentials evoked by the expiratory depolarization. Depending on the state of the preparation, hindlimb muscle nerves showed various patterns of discharge, most often firing in early expiration (post-inspiration, pI) or inspiration. In such a state, pI drive potentials were seen, often with a negligible amplitude until depolarizing currents were applied (cf. Kirkwood et al. 2002). The amplification was therefore large (≧20×) in such cells, which also showed prominent plateaux. We conclude that the expression of PICs in spinal motoneurones varies widely, according to the muscle innervated and the state of the animal.