Flunitrazepam 0.5, 1.0 or 2.0 mg was given by the oral or i.m. routes to groups of volunteers and its effccts compared. Plasma concentrations of the drug were estimated by gas-liquid chromatography, in a smaller number of the subjects. The most striking effect was sedation which incraned with the dose, 2 mg producing deep sleep although the subjects could still be aroused. The cffects of i.m. administration were apparent earlier and sometimes lasted longer than those following oral adminiatration. Dizziness was lees marked than sedation, but increased with the dose. There was pain on i.m. injection of flunicrazepam significantly more often than with isotonic saline. Plasma conccntrations varied with dose and mute and corresponded qualitatively with the subjective effects. The drug was still present in measurable quantities aftar 24 h even with the smalleat dose.