Tetracycline chloride dissolved in saline was injected intravenously to seven cows. Two doses of tetracycline were used: 5 and 10 mg/kg b.wt, and the injections were given over a period of either 10, 60 or 300 sec.A number of the cows collapsed shortly after the injection was completed, usually when the 10 mg dosage was given in 60 sec. When the same dose was given over a period of 5 min none of the cows collapsed. A more or less pronounced drop in blood pressure could be detected during or shortly after the injection; in those cows which collapsed the blood pressure fell almost to zero. The predominant change in pulse rate in connection with the tetracycline administration was a decrease which could be quite marked, pulse rates falling as low as 10–20 per min. Simultaneously with these changes in blood pressure and pulse rate severe abnormalities in ECG could be observed. Pre‐treatment with a normal therapeutic dose of calcium borogluconate intravenously prevented collapse in the cows and diminished the drop in blood pressure associated with an ensuing tetracycline injection.It is concluded that intravenous injection of tetracycline is hazardous, but that collapse can be avoided by giving the injection very slowly over a period of no less than 5