Alternative substrates other than glucose could be used by the brain. In this study we hypothesized that lactate and ketone bodies can provide a significant portion of oxidative brain substrates in insulin-dependent diabetes mellitus (IDDM). Six control (C) and six insulin-treated streptozotocin diabetic (IDDM) dogs were studied during euglycemia (EU) and acute insulin induced hypoglycemia (HYPO). During EU for similar plasma glucose concentration (5.5 ± 0.4 v 5.2 ± 0.2 mmol/L in IDDM dogs showed a higher baseline lactate concentration (1.5 ± 0.25 v 0.74 ± 0.10 mmol/L; P < .05). The ketone body concentrations were also increased in IDDM dogs but this increase was not statistically significant. The brain glucose uptake was 6.9 ± 0.6 μmol/kg/min in C and 5.4 ± 0.7 in IDDM. Lactate was released by the brain both in IDDM dogs (11.36 ± 1.8 μmol/kg/min) and in C dogs (3.87 ± 0.9; P < .05). The brain ketones rate of disappearance (Rd) was 0.3 ± 0.05 μmol/kg/min in IDDM dogs and 0.19 ± 0.08 in C dogs. During HYPO the glucose uptake across the brain was 2.88 ± 0.7 μmol/kg/min in IDDM and 3.12 ± 0.5 in C dogs. We observed an overall brain lactate release (3.21 ± 1.7 mol/kg/min) in C dogs and a net uptake (13.44 ± 1.1; P < .01) in IDDM (P < .01). The brain ketones Rd was 0.1 ± 0.2 μmol/kg/min in IDDM and 0.1 ± 0.1 in C dogs. These results suggest that (1) lactate uptake by the brain is increased during acute hypoglycemia in insulin dependent diabetes; (2) ketone bodies represent a fuel of minor importance in the brain during intensified insulin therapy and during acute hypoglycemia. During hypoglycemia these results could substantiate the role of lactate as source of energy for the brain other than glucose.