Anesthetized dogs (pentobarbitone 25 mg/kg + 3 mg/h) with a Teflon catheter inserted into the femoral artery for pressure and heart rate measurement and blood sampling were studied. Extradural puncture was performed at L[lumbar]7-S[sacral]1 under X-ray control and 2% lignocaine (1 ml/min) infused for .apprx. 10 min. During extradural block a decrease in arterial pressure was accompanied by a simultaneous decrease in plasma noradrenaline concentration (NA, norepinephrine) and a compensatory increase in plasma renin activity (PRA), which became significant 15 min after commencing the block when the arterial pressure was at the minimum. Dihydroergotamine (DHE) rapidly reversed the hypotension with no change in heart rate and a decrease in PRA. Hypovolemia (blood loss 5 ml/kg) was induced before and during block, respectively, in 2 further experimental groups. NA and PRA increased under hypovolemia without extradural block, but only PRA increased with hypovolemia and a block. The effects of DHE in the later studies were comparable to those observed in the initial study.