A double-blind, controlled trial is described in which a total of forty hospital in-patients suffering from severe post-operative pain were randomly allocated to treatment with one of two non-steroidal anti-inflammatory drugs, namely, either indoprofen which has a short half-life (two-hours) or naproxen which has a long half-life (thirteen hours). The drugs were administered orally on a single-dose basis. The doses used in this way were 300 mg of indoprofen or 250 mg of naproxen. Patients scored the severity of their pain on a five-point scale and these scores were recorded prior to and at fixed time intervals up to eight hours following administration of medication. No significant differences emerged between the two test drugs and the duration of the response was also found to be similar for the two compounds despite their very different plasma half-life values.