Ascending aortic blood velocity was measured in the baboon (Papio ursinus) by using continuous wave Doppler ultrasound. The blood flow parameters thus obtained were compared to those by the standardized radionuclide technique. It appears that, due to the anatomical position of the ascending aorta and brachiocephalic trunks in relation to the ultrasound beam, Doppler ultrasound does not provide an accurate method of measuring aortic blood velocity in the baboon, which could be the reason for the poor correlation of the results from the two techniques.