The soleus (Sol) and medial and lateral gastrocnemii (GM and GL) behave differently during various movements, but no attempt has been made to disclose any distinct activation of these muscles during perturbations of upright stance. Therefore the pattern of activation of the three triceps surae (TS) muscles and of the tibialis anterior (TA) was studied in normal subjects following rotational and linear displacements of a movable platform. The effect of "postural set" on the responses was also studied while holding onto a frame. In free-standing subjects, TS stretches (upward tilt, UT and backward translation, BT) evoked a large short latency response (SLR) in Sol; smaller SLRs were sometimes induced in GM or GL. A medium latency response (MLR) was consistently present in both or only one gastrocnemii. On the average, the amplitude and the frequency of occurrence of the responses were distributed as follows: SLR, Sol greater than GL greater than GM; MLR, GM greater than GL greater than Sol. The type of perturbation did not affect the latency of all TS muscle responses, but the duration, amplitude and frequency of MLRs were larger during BT than UT. MLRs were followed by an antagonistic reaction (AR) in the TA, larger and more frequent during UT than BT. TA stretches (downward tilt, DT and forward translation, FT) induced a TA MLR, with duration and area larger in FT than DT. ARs occurred in one or more muscles of TS, being larger and more frequent in Sol. Under the conditions of stabilized stance, SLRs were not affected, but all the MLRs and ARs were much reduced in amplitude. The analogies between TS and TA MLRs (frequency of occurrence, latency and suppression under stabilized condition) suggest a common underlying mechanism and a similar postural role. On the other hand, all the TS responses are unequally distributed in the individual muscles and in the various subjects. This recommends caution in drawing conclusions in their absence or from their susceptibility to postural set in patients, when only one muscle of TS is being recorded.