Forty-eight intensely fattened weaned lambs, divided into 2 groups were used to test the prophylactic effect of lasalocid with regard to rumen acidosis. The animals were offered complete mixtures ad libitum, containing 80 per cent concentrates and 20 per cent cut meadow hay. The weaned lambs of the test group were given the preparation "Avatek' (with 15 per cent lasalocid) in the course of 60 days with the feed at the rate of 250 g per ton of the complete mixture. Periodic biochemical and hematologic investigations on the animals of the two groups were carried out, following up their body development. The animals of the test group showed higher level of alkaline reserves, volatile fatty acids, and pH values of the rumen content and lower level of lactic acid in the blood and rumen liquid. By the 45th day of the experiment certain changes in the biochemical indices took place (rise of lactic acid and drop of alkaline reserves) as well as arrest of the weight development in the test animals. The average daily gain of the test group over the entire experimental period was 7.74 per cent higher than that of the control animals.