This experiment was conducted to compare the fatty acid composition in the backfat and the levels of testosterone and androstenone in the serum of boars, barrows and gilts. The results obrained are summarized as follows : 1. The contents of palmitic acid and stearic acid in the backfat of barrows increased as their body weight increased and the levels of them were higher in barrows than in boars. However, the contents of oleic acid and linoleic acid were lower in barrows than in boars. 2. The levels of testosterone in boars at 70, 90 and 110 kg of body weight were 3.25, 5.66 and 4.5ng/ml respectively, while those of barrows or gilts were 0.01-0.43ng/ml. 3. The level of androstenone in boars at 110kg of body weight was higher(1.04ng/ml) than that at 70kg of body wight(0.44ng/ml), but there was little difference in the levels of it by body weight or age in barrows and gilts.