1. In biopsy samples of the lateral part of m. quadriceps femoris of 49 obese and 14 lean persons the activities of the following enzymes were investigated: triosephosphate dehydrogenase (TPDH), glycerolphosphate: nad dehydrogenase (GPDH), lactate dehydrogenase (LDH), hexokinase (HK), malate: NAD dehydrogenase (MDH), citrate synthase (CS) and hydroxyacyl-CoA dehydrogenase (HOADH). 2. The muscles of obese had an increased activity ratio of TPDH to CS and to HK, respectively, caused in muscles of female obese subjects by an increase of TPDH activity, in those of obese men rather by a decrease of CS and HK activities. 3. Cluster analysis brough to light the existence of three major groups. Group 1 (low activity-low LDH group), consisting of muscles of female obese subjects only, exhibited low activities of all enzymes investigated, that of LDH being so low as to possibly induce a serious deficiency of anerobic metabolism under working conditions. Group 2 (medium enzyme activity group) was characterized by medium enzyme activities, similar to that of lean controls (included in this group). This consisted of subjects of both sex. Group 3 (high enzyme activity group) consisted of obese of both sex. It was distinguished by high enzyme activities, especially of LDH. It is suggested that the groups of similar enzyme activity patterns might reflect different stages, types and/or genesis of obesity.