In monkeys generating torques about the wrist we investigated changes in the excitability of pyramidal tract (PT) axons, measured as the probability of evoked antidromic responses in motor cortex with constant juxtathreshold stimuli delivered in the brain stem. When PT stimuli were delivered 2-20 ms after an orthodromic action potential in the PT neuron, the excitability of axons was elevated, with a characteristic post-spike time course. Excitability peaked at a post-pike delay of 7.0 +/- 2.7 ms (n = 33). Axonal thresholds typically dropped to 80-90% of the unconditioned values (obtained for stimuli with no preceding spike). Controlling for such post-spike threshold changes by delivering stimuli at fixed post-spike delays, we found that excitability of many PT axons also fluctuated with the wrist responses, being slightly higher during flexion or extension. The place of movement in which excitability increased had no consistent relation to the phase of movement in which the PTN fired. Task-related threshold changes were also seen in PTNs whose discharge was not modulated with the wrist response. Delivering a subthreshold conditioning stimulus also increased the excitability of most PT axons to a subsequent test stimulus. Such post-stimulus changes may be mediated by the effects of adjacent fibers activated by the conditioning stimuli. The post-spike and post-stimulus changed added in a nonlinear way. All three types of threshold change may be mediated by a common mechanism: changes in the ionic environment of the axon produced by activity of the axon itself or its neighbors.(ABSTRACT TRUNCATED AT 250 WORDS)