An introductory survey is given of the cellular physiology of motoneurones (MNs). Steady driving currents, applied to individual cells through microelectrodes, may be used for determining such key parameters as the range of possible discharge rates and the shape and steepness of the curve relating discharge frequency to current intensity (f-I relation). Quantitatively, MN properties may vary considerably between animal species and between cells innervating different types of muscle fibres. Central synapses impinging upon MNs often simply provide "driving" currents, altering MN discharge rate largely in accordance with the f-I relation. In addition, metabotropic synapses may have "MN-modifying" effects, altering MN membrane and activation properties in various ways. Studies of MN firing and response patterns in normal and pathological motor behaviour is essential for evaluating the functional role of short- and long-term modifications of MN properties.