Abstract In 15 unpremedicated dogs, an inverse relationship between age and cerebral metabolic rate (CMR O 2 ) was observed. In dogs estimated to be 0.5 to 1.5 and 1.5 to 3 years of age, the mean CMR O 2 values (5.27 and 4.85 ml./100 gm./min.) were significantly greater than that value of dogs more than 3 years old (4.22 ml./100 gm./min.). No difference in metabolic pathways between the groups was observed. The relative magnitude of the effect of age on CMR O 2 is similar to that reported for man (assuming a 1:7-year ratio). Such an effect provides a significant potential source of error when comparing individual cerebral metabolic studies in animals of a different or an unknown age.