Nitrogen at high pressures and anesthetics increase lipid monolayer surface pressure and in turn modulates monolayer associated lipolytic enzyme activity that could alter membrane lipids. We tested the hypothesis that nitrogen at pressures of 5 and 10 megapascals (MPa) and pentobarbital induce alterations in synaptosomal membrane phospholipid and free fatty acid (FFA). Rat cortical synaptosomes in Krebs-Henseleit buffer were placed in steel chambers and incubated for four hours at 37 degrees C: at 5 or 10 MPa of O2/balance N2; at one 0.1 MPa on room air, and with 10 mg pentobarbital. Free fatty acids (FFA) were quantified by thin-layer and gas chromatography, and neutral and acidic lipids by high-pressure thin layer chromatography and protein by Biorad colorimetric assay. Statistical analyses were by ANOVA and posthoc analysis by Neuman-Keuls and Kruskal-Wallis tests at p < 0.05. Sphyngomyelin, phosphatidylcholine, phosphatidylethanolamine, cerebroside and cholesterol were unchanged by 5 and 10 MPa nitrogen and pentobarbital. Free fatty acids (16:00, 18:00, 18:01, 20:00, 22:0, 22:01 and 24:01) at 10 MPa were reduced compared to 5 MPa (p < 0.05) but unaffected by pentobarbital. The decrease in synaptosomal membrane FFA at 10 MPa suggests attenuated hydrolysis of membrane phospholipids without detectable alterations in membrane phospholipid composition.