A modified headspace gas chromatographic method for analysis of formate in biological fluids is described. Serum or whole blood specimens were methylated in the presence of concentrated sulfuric acid and sodium propionate (internal standard) dissolved in methanol. Derivatization was performed at 35 degrees C for 30 min before injection (0.5 mL headspace) onto a Hallcomid-Carbowax packed GLC column. Using serum or aqueous standards, the method was linear from 5 to 100 mg/dL. The limit of detection was 2.5 mg/dL. Day-to-day precision was less than 5% (CV) at 54 mg/dL formate. Formate and methanol were analyzed in 3 methanol poisonings, two of which were fatal. Formate analysis is considered important in any patient with suspected methanol poisoning who presents for medical assistance with metabolic acidosis. The extent of ocular toxicity correlates better with formate concentration than with methanol concentration.