The effect of large doses of doxycycline (Vibravenös) and rolitetracycline on blood pressure, heart rate and ECG was investigated in 36 conscious minipigs. The antibiotics were infused into the v. tibialis at doses of 16, 24 and 32 mg/kg at injection periods of 4, 6 and 8 min. The dosage applied was about 10-20 times higher than the therapeutical single dose of doxycycline and about 4-8 times higher than the single dose of rolitetracycline. 16 mg doxycycline or rolitetracycline/kg resulted in a minor short-lasting increase of blood pressure and extension of the P-Q interval in the ECG. The definitely pathological P-Q value of 160 ms was not observed in any of the cases treated with doxycycline, whereas rotitetracycline led to clearly pathological P-Q values in 2/6 of the cases, and to AV block in 4/6 of the cases. Neither after doxycycline nor after rolitetracycline did the increased dose of 24 and 32 mg/kg result in an enhanced effect on circulation. However, both the blood pressure and the P-Q values following 32 mg rolitetracycline/kg significantly exceeded those following 32 mg doxycycline/kg. These trials suggest that with regard to circulatory tolerance there are no objections to increasing the usual therapeutical dose of intravenous doxycycline if the injection time is adequately extended.