Muscles from three patients were obtained during continuous ischemic periods and were investigated ultrastructural changes to know the reversible tourniquet time. Tourniquet was applied for periods lasting between 60 minutes and 90 minutes in the human and muscle biopsies were taken from 10 minutes to 90 minutes ischemic period and at the same periods after release of the tourniquet. All specimens were investigated with electron microscopy and the results were as follows: In 10 minutes tourniquet period, loss of cristae in few mitochondria were observed. In 30 minutes tourniquet period, partial swelling, loss of cristae and myelin figure in many mitochondrias and vacuolar dilatation in T-tubule and sarcoplasmic reticulumn were observed. In 60 minutes tourniquet period, above change were observed in nearly all mitochondrias and total number of mitochondrias were decreased. In 90 minutes tourniquet period, above changes were more distinct. 20 minutes after release of 90 minutes tourniquet period, small sized new mitochondria were appeared and abundant electron dense granule in their matrix and sarcoplasmic reticulumn were visible. 60 minutes after release of 90 minutes tourniquet period, the majority of damaged mitochondrias were replaced by small sized new mitochondrias and electron dense granule were abundant in sarcoplasmic reticulumn. In human, the changes on the ultrastructure were reversible for at least a succeeding 90 minutes ischemic periods.