The active components of the quadrupedal diagonal stance response to rapid removal of the support from beneath a single limb were studied in cats to further define the mechanisms that trigger and generate the response. We recorded EMG activity from lateral gastrocnemius and tibialis anterior muscles in awake, behaving cats while they stood on an hydraulic posture platform. By dropping the support from beneath a single limb, we evoked the diagonal stance response, with its characteristic changes in vertical force and EMG patterns. As the animal responded to this drop, a second perturbation of posture was then presented at intervals of 10 to 100 ms following the first. The second perturbation, which consisted of dropping the support from beneath the two limbs that were loaded as a result of the initial limb drop, made the first response biomechanically inappropriate. The EMG responses observed in both muscles during paired perturbations were triggered by the somatosensory events related to the perturbations. Muscle responses that were appropriate for the first perturbation always occurred with amplitudes and latencies similar to control trials. This was true even when the second perturbation occurred 10-20 ms after the first, that is, when this perturbation either preceded or was coincident with the response to the initial limb drop. The EMG responses that were normally associated with the second perturbation were delayed and/or reduced in amplitude when the time interval between perturbations was short. As the inter-perturbation interval was lengthened beyond 60-100 ms, however, EMG responses to the second perturbation were unaffected by the occurrence of the first perturbation. When the hindlimb containing the recording electrodes was dropped as part of the second perturbation, a myotatic latency response was observed in tibialis anterior. The amplitude of this response to the second perturbation was greater than controls when this displacement was presented during the period between initiation of the first perturbation and execution of the response to it. When the second displacement was presented after execution of the first response began, the amplitude of the myotatic response was reduced below control levels. While the results do not preclude the possibility that these "automatic" postural responses are segmental or suprasegmental reflexes, they support the hypothesis that the active component of the response to drop of the support beneath a single limb is centrally programmed and that the appropriate response can be triggered very rapidly by the somatosensory information signalling the perturbation.