We investigate the mechanisms underlying the redox switch/redox coupling hypothesis by characterizing the competitive consumption of glucose or lactate and the kinetics of pyruvate production in primary cultures of cortical neurons and astrocytes from rat brain. Glucose consumption was determined in neuronal cultures incubated in Krebs ringer bicarbonate buffer (KRB) containing 0.25–5 mM glucose, in the presence and absence of 5 mM lactate as an alternative substrate. Lactate consumption was measured in neuronal cultures incubated with 1–15 mM lactate, in the presence and absence of 1 mM glucose. In both cases, the alternative substrate increased the Km (mM) values for glucose consumption (from 2.2 ± 0.2 to 3.6 ± 0.1) or lactate consumption (from 7.8 ± 0.1 to 8.5 ± 0.1) without significant changes on the corresponding Vmax. This is consistent with a competitive inhibition between the simultaneous consumption of glucose and lactate. When cultures of neurons or astrocytes were incubated with increasing lactate concentrations 1–20 mM, pyruvate production was observed with Km (mM) and Vmax (nmol/mg/h) values of 1.0 ± 0.1 and 109 ± 4 in neurons, or 0.28 ± 0.1 and 342 ± 54 in astrocytes. Thus, astrocytes or neurons are able to return to the incubation medium as pyruvate, a significant part of the lactate consumed. Present results support the reversible exchange of reducing equivalents between neurons and astrocytes in the form of lactate or pyruvate. Monocarboxylate exchange is envisioned to operate under near equilibrium, with the transcellular flux directed thermodynamically toward the more oxidized intracellular redox environment. © 2007 Wiley-Liss, Inc.