In anaesthetized cats, thresholds for long (rheobase) and brief duration current pulses have been obtained from spinal motoneurones and compared with other cell parameters and membrane properties. Rheobase showed only weak over-all relationships with conduction velocity and with cell size, estimated as the total capacitance of individual motoneuronal equivalent cylinders. Rheobase showed a clear tendency to vary inversely with after-hyperpolarization (a.h.p.) duration and was strongly correlated with the input conductance and with the inverse of the membrane time constant. However, the range of rheobase current exceeded that of input conductance by almost a factor of 2. Part of this range discrepancy arose because threshold depolarization tended to increase with rheobase current. Thus, among motoneurones grouped according to rheobase magnitude (three groups), those within the lowest rheobase group had threshold depolarizations about 6 mV on average lower than those within the highest rheobase group. Even though this difference was not directly related to resting potential differences between the groups, further analysis suggested that it may have arisen secondarily to impalement-induced depolarization. The finding that experimentally estimated threshold depolarizations in individual motoneurones were generally larger than those predicted by the product of input resistance and rheobase indicated that a subthreshold rectification process also contributed to the range of rheobase. The difference was largest in the low-rheobase group and smallest in the high-rheobase group. Because these differences were proportional to the differences in input resistance between the separate motoneurone groups, it is suggested that the magnitude of the current underlying the rectification process does not differ systematically among motoneurones. Within groups of motoneurones classified on the basis of rheobase or a.h.p. duration, significant correlations existed between rheobase current and input conductance. An analysis of variance indicated that even within such functional subgroups of motoneurones, rheobase was appreciably better correlated with membrane time constant than with estimated cell size. Although showing a range approximately half that of rheobase, the brief current threshold was similar to rheobase in its relations with total cell capacitance, a.h.p. duration and the inverse of membrane time constant.(ABSTRACT TRUNCATED AT 400 WORDS)