An experiment was designed to study the clinical effects of different levels of carbadox, cyadox and olaquindox in the ration on health, weekly weight gain and feed conversion in pigs. Four different carbadox and olaquindox (25, 50, 100 and 200 ppm) levels and five different cyadox (25, 50, 100, 200 and 400 ppm) levels were tested in groups of 6 pigs during 6 weeks. The 13 groups were compared with a control group fed on the same diet with only vehicle. After one week the first clinical sign, a high faecal dry matter (FDM) content, was observed in the 200 ppm carbadox group, followed by the 100 and 50 ppm carbadox, the 400 and 100 ppm cyadox, and the 200 and 100 ppm olaquindox groups two weeks later. A second clinical sign, urine drinking from the floor or from pen-mates, was observed in the same pens, occurring in the same sequence. The third important clinical sign, a decreased abdominal volume, was also observed in almost the same sequence, however, in the 50 ppm olaquindox and cyadox groups this clinical sign was not observed. Average weekly weight gain was significantly decreased in the higher carbadox and olaquindox groups. Weight gain was significantly increased in the 200 ppm cyadox group. Hematocrit values were significantly increased in the 200 and 100 ppm carbadox groups only. From this study one may conclude that, within the dosages used, carbadox is more harmful than olaquindox for pigs, and it seems that cyadox is harmless for pigs in dosages up to 400 ppm.