The imidazobenzodiazepine midazolam (MDZ), along with its 1-hydroxy and 4-hydroxy metabolites (1-OH-MDZ, 4-OH-MDZ) can be simultaneously quantitated by electron capture gas-liquid chromatography. After addition of diazepam as internal standard, alkalinized plasma samples are extracted into benzene-isoamyl alcohol. The organic extract is separated, evaporated to dryness, reconstituted, and chromatographed using 3% SP-2250 as the liquid phase. The identity of 4-OH-MDZ, not previously detected in human plasma in unconjugated form, was verified by negative-ion chemical ionization mass spectroscopy. After single oral doses of MDZ administered to humans, concentrations of MDZ and 1-OH-MDZ (alternatively named 1-hydroxymethyl midazolam) were similar, and both compounds were eliminated from plasma in parallel. Concentrations of 4-OH-MDZ were considerably lower, but this metabolite was also eliminated in parallel with the parent compound.