The recommended rates for closed-chest cardiac compressions during CPR are based on physiologic variations with increasing age rather than experimental data. Using puppies, we compared mean arterial pressure, cardiac index (CI), and cerebral blood flow (CBF) at a slow (40/min, group 1) and a rapid (120/min, group 2) rate. CBF was measured in all experiments by N2O uptake before and during CPR from cardiac arrest induced with KCI. Both CI and CBF were measured in half of the subjects with radiolabeled mcrospheres. Groups 1 and 2 were similar in terms of baseline mean weight, mean arterial pressure, CI, CBF, and arterial and venous blood gases. During resuscitation for group 1, the mean CI was 221 ml/min m2, and the mean CBF was 4 ml/100 gċmin determined by N2O uptake (2 ml/100 gċmin by microspheres); in group 2, the respective means were 248 ml/minċm2and 4 ml/100 gċmin with both techniques. The values for mean CI and CBF (N2O or microspheres) did not differ significantly at either rate of compression. The N2O uptake and microsphere techniques for measuring CBF correlated closely during spontaneous cardiac contractions and mechanical chest compressions. We conclude that a) closed-chest cardiac compressions at either rate studied provided inadequate CBF, and b) the N2O uptake and microsphere techniques give similar measures of CBF under conditions of normal and low flow.