Eighteen of 91 seven- to nine-month-old Belgian white and blue double-muscled male fattening cattle developed typical signs of shipping fever. They were all injected intramuscularly once a day for three days with 5 mg/kg of enrofloxacin, and in addition nine selected at random were injected intramuscularly five times at 12 hour intervals with 0.1 mg/kg of metrenperone, a 5-hydroxytryptamine blocker, the other nine receiving a placebo. During the outbreak of shipping fever metrenperone showed effective antipyretic properties, and all the calves treated with it made a complete recovery. Moreover, during the 360 day fattening period following the outbreak, the cattle treated with metrenperone gained on average 45.4 kg more weight than the control cattle.