BCNU levels were determined by differential pulse polarography in 12 patients with lung cancer. A simple assay method for submicrogram quantities of nitrosoureas is described. The half-life of BCNU added to plasma in vitro was 1413 mins at 0 degrees C and 12 mins at 37 degrees C during incubation. BCNU kinetics in blood of treated patients was calculated using the two-compartment open model; the half-life of the drug was 1.4 mins in the first phase and 17.8 mins in the second. Mean distribution volume was 2.59 liters/kg and plasma clearance was 16.7 ml/min/kg.