Studies were made of the ability of erythrocytes from 18 cord bloods to reduce methemoglobin in vitro. Erythrocytes of cord blood reduced significantly less methemoglobin than did those of adult controls in the presence of lactate, lactate and methylene blue, or glucose. These findings are interpreted as evidence of a transient deficiency of either DPNH-dependent methemoglobin reductase or of one of the enzymes responsible for the generation of DPNH in the erythrocyte. The severity of this defect was highly variable. In contrast, TPNH-dependent reduction of methemoglobin by erythrocytes of cord blood in the presence of glucose and methylene blue was only infrequently deficient. Evidence is presented to suggest that this deficiency, when present, is due at least in part to deficiency of generation of TPNH. The frequency of deficient DPNH-dependent reduction of methemoglobin may explain the ease with which young infants develop methemoglobinemia.