Abrupt unloading of the human forearm can produce muscle activations in the triceps brachii as a result of stretch (Paulignan et al. 1989). The stretch reflex of this muscle occurring at a latency of approximately 50 ms has a transcortical component (Thilmann et al. 1991) and increased response amplitudes may indicate increased levels of cortical excitability (Day et al. 1991). With local ethics committee approval we investigated the effect of both an external and an internal timing signal on the amplitude of the long-latency stretch reflex of the triceps brachii. Eight healthy male subjects were seated upright with their right upper arm fixed to a support. An electromagnet attached underneath their right wrist supported a 1.5-kg load. They were instructed to maintain their loaded forearm in the horizontal plane, creating an elbow angle of 115°. The subjects performed sixty trials in each of three randomised unloading conditions. First, in the control situation the unloading was imposed and unpredictable in time. Second, the unloading was imposed but signalled by a tone 300 ms prior to release. Third, the unloading was performed voluntarily as the subject pressed a switch that initiated the load release after a delay of 300 ms. The mean rectified electromyographic (EMG) activity from the lateral head of the right triceps brachii muscle was measured in the last twenty trials in the following time bins: 100 to 0 ms prior to the load release and at 50 to 95 ms after the load release. The mean elbow rotation velocity was measured between the moment of load release and the onset of the long latency stretch reflex 50 ms later. The results are presented as mean ± SD and values significantly different from the unpredictable condition values are shown as * P<0.03 (repeated measures ANOVA with LSD post hoc). In the imposed unpredictable condition, the EMG activity at the time of the expected long latency stretch reflex dropped below baseline values to 6.6 ± 2.9 μV. However, in the imposed tone and voluntary conditions we observed long latency stretch reflex amplitudes of 14.4 ± 7.9 μV* and 22.4 ± 16.4 μV*. We interpret this as an increase in gain of the reflex as the mean elbow rotation velocity was similar in all three conditions. (Unpredictable 91.8 ± 15.9°.s-^null1null, Tone 89.6 ± 12.1°.s-^null1null, Voluntary 85.6 ± 9.5°.s-^null1null). We conclude that both an internal (voluntary) and external (tone) timing signal had the effect of increasing the gain of the long latency stretch reflex. This may be due to the afferent signals resulting from the perturbation reaching a motor cortex made more excitable by the previous timing signals.