The applicability of radioactive microspheres in measuring cardiac output (CO) during haemorrhagic shock in the dog was studied. The COs thus determined were compared with CO values obtained simultaneously by the Fick method. In 13 animals simultaneous CO measurements by the two methods were performed prior to and after initial bleeding (down to a mean arterial pressure of 40 mmHg), at the termination of the hypotension period, and 5 and 60 min following blood reinfusion. An excellent correlation (r = 0.97) between the two methods was found for all determinations combined. Furthermore, during each of the five phases of the shock procedure no difference was observed between the two methods of measuring CO and the correlation varied from moderate to excellent (r = 0.61-0.94). The results suggest that the microsphere method has a precision and validity comparable to those of conventional methods in measuring CO in situations when it is extremely low, as in haemorrhagic shock, in addition to having high accuracy in normal conditions.