Propylene glycol (PG) concentrations were measured in 35 sera and eight cerebrospinal fluids (CSF) from five patients receiving intravenous medications containing PG as a vehicle. Serum concentrations ranged from 6 to 711 mg/L, and CSF concentrations from 11 to 566 mg/L. The CSF concentrations were as high as 85% of the serum concentrations. In several instances, PG was also measured in urine and pleural fluid. Lactate concentrations ranged up to 24.1 mEq/L (217 mg/dL) and showed statistically significant correlation (r = 0.921) with the PG concentrations in serum (p less than 0.01). The PG:creatinine clearance ratios in two patients were 35.5% and 48.8%. Serum half-lives in two patients were 4.7 and 5.6 hrs. Propylene glycol administration may be an important cause of lactic acidosis in the hospitalized patient.