To specify pharmacological interferences between droperidol and fentanyl an experimental study was carried out on mice. The analgesic kinetic was studied after the administration of the two drugs in a concentration ratio between 25/1 and 150/1. RESULTS Fentanyl is a potent analgesic with a short time effect. It has very good correlations between dose effect and time effect. Droperidol does not have a analgesic effect. The behaviour of the two drugs depends on the way they are administered, either simultaneously or consecutively: simultaneous administration of the two drugs increases the level and duration of analgesia: nalaxone reverses analgesia, a fentanyl injection given 5 minutes before a droperidol injection produces the same analgesia as fentanyl alone, a droperidol injection given 5 minutes before a fentanyl injection induces a level of analgesia lower than fentanyl alone. Droperidol appears to reverse the fentanyl analgesia.