Experiments with pigs of the White Bulgarian Improved breed of initial weight of 25 kg revealed that the oral administration of salinomycin at the rate of 50 ppm (from the beginning of treatment up to the 61st day) and 25 ppm (from the 62nd day up to the 117th day) with the feed led to the increase in weight gain by 10.9 per cent and in feed conversion by 7.4 per cent by the end of the experimental period. The effect of nutrition was manifested regularly in the course of both periods of fattening. The slaughter weight also rose under the effect of salinomycin. It should be noted that higher rates of treatment--100, resp. 50 ppm--for the respective periods led to the reduction of the stimulant action of salinomycin. Under the conditions to the investigation nitrovin did not show a clearly manifested nutrition effect.