Differences in skeletal muscle characteristics, metabolic profiles and functional performance between males and females were investigated using young (15--24 yrs) male and female twins as subjects. The comparison included such variables as anthropometry, muscle strength, mechanical power, maximum oxygen uptake, electrical activation of muscle, muscle fibre composition (m. vastus lateralis), and activities of several skeletal muscle enzymes. The results disclosed the following primary differences between males and females: In the various functional tests the performance of females was from 61.1 to 84.6% of that in males; distribution of slow twitch fibres in m. vastus lateralis of the females (49.1 +/- 7.7%) was lower (p less than .05) than that of the males (55.9 +/- 11.9); activities of enzymes Ca2+ stimulated ATPase, CPK, phosphorylase and LDH1a leads to py were higher (p less than .05--0.1) in the males, whereas the distribution pattern of LDH-1 isozyme was higher (p less than .05) in the females. A pronounced difference between the two groups was a almost 100% longer rise time of isometric force in females. It is concluded that the males as compared to the females demonstrate higher aerobic and strength performance capacity, more efficient neuromotoric output during contraction, more slow twitch muscle fibres and more pronounced contractile and glycolytic profiles in the skeletal muscles.