This report extends the finding that serum protein(s) (0.5 mg/ml) caused a 50% decrease in the rate of glucose oxidation by dissociated brain cells with only marginal effects on the oxidation of other substrates. Since dissociated cells represent a heterogeneous population, studies were initiated to determine the effect of serum on the rates of substrate oxidation by isolated synaptosomes and cultured rat brain astrocytes. Experiments revealed that the addition of 5% serum v/v to the reaction mixture resulted in a decrease in the rate of 14CO2 production from [6-14C]glucose by isolated synaptosomes by more than 70%. In contrast, the addition of 5% serum had little or no effect on the 14CO2 production from [U-14C]glutamine by the synaptosomes and only marginal effects (20-25%) on 14CO2 production from [U-14C]lactate and 3-hydroxy[3-14C]butyrate. The effect of serum on the rates of substrate oxidation were similar for synaptosomal preparations obtained from adult animal brains or 18-day-old rats, except that with the latter preparation, 14CO2 production from 3-hydroxy[3-14C]butyrate was more attenuated by the presence of serum than with the former synaptosomal preparation (50 vs. 25%). In contrast to the results with synaptosomes, the presence of 5% serum enhanced the rates of 14CO2 production from [6-14C]glucose, 3-hydroxy[3-14C]butyrate and [U-14C]lactate by 61, 35 and 69%, respectively, in cultured rat brain astrocytes. However, this enhancement did not occur when the cells were grown in chemically defined media or when dibutyryl cAMP was added to the media.(ABSTRACT TRUNCATED AT 250 WORDS)