Two groups of female lambs--'hay' and 'concentrate'--of 10 animals each, equated in terms of age, breed, and live weight were given different rations from the age of two weeks to one year. The hay group was offered roughage ad libitum and concentrates up to 25 per cent of the total amount of feed. The concentrate group was given concentrates ad libitum and roughage up to 25 per cent of the total amount of the feed consumed. The body mass of the animals was measured every month. There was no differences in the live weight between the two groups up to the age of 6.5 to 7.5 months. Later on the concentrate group showed 1.52 kg higher weight. This difference rose with each following month, and at the age of one year it reached 10 kg. The curves of body mass growth for the two groups showed the same pattern up to the age of 7.5 months (the period of intense growth). Then the curve of the hay group slowed down its course, while that of the concentrate group continued its growth (a period of delayed growth and enhanced fattening, respectively). The growth of body mass exhibited a characteristic rhythm which was manifested regardless of the type of ration given ad libitum.