To investigate the phasic activity of the lower extremity muscles during up- and downslope walking, five muscles of ten healthy men were examined by telemetered electromyography (EMG). The muscles were the tibialis anterior (TA), gastrocnemius (Gc), rectus femoris (RF), semitendinosus (St) and gluteus maximus (GM). The inclination of the slope was 3, 6, 9 and 12 degrees. EMG of the muscles and the time factors of a walking cycle were recorded by a 12-channel polygraph simultaneously. In upslope walking, the duration of TA, St and GM activity was longer and that of RF activity was shorter than in level walking. The phasic pattern of Gc in upslope walking was the same as in level walking. In downslope, the duration of Gc and RF activity was longer than on the level. St showed biphasic activities. The phasic pattern of TA and GM was nearly the same as in level walking. The phasic activity of the muscles altered with an inclination over 6 degrees in upslope, and over 3 degrees in downslope. The findings indicate that the muscles stabilize the knee and ankle joint much more in slope walking than in level walking, and in slope walking they also exert themselves to elevate or lower the body weight.