To define dose- and time-response properties for in vivo inhibition of renal prostaglandin (PG) synthesis, aspirin, diclofenac sodium, indomethacin and d-naproxen were injected intravenously in different doses to unanaesthetized rabbits. After 30 min. the animals were killed and the post mortem accumulation of PGE2 and PGF2alpha in the renal medulla was determined by mass fragmentography. In control animals, the accumulated levels of PGE2 and PGF2alpha in the medulla were 9.2+/-2.2 (S.D.) and 1.5+/-0.6 microgram/g, respectively. Dose-dependent inhibition was demonstrated with all the drugs. The ED95 was for aspirin 15 mg/kg, for diclofenac sodium 1.5 mg/kg, for indomethacin 1.5 mg/kg and for d-naproxen 5 mg/kg. The duration of inhibition was studied by radioimmunoassay in anaesthetized rabbits by following the urinary excretion of PGF2alpha and PGE2 after an intravenous injection of solvent or test drug in doses twice the ED95. For three hours following aspirin, diclofenac sodium, indomethacin and d-naproxen, the decreases in urinary excretion of PGF2alpha ranged from 64 to 88, 87 to 95, 64 to 90 and from 40 to 77 per cent of the control, respectively, and the decreases in PGE2 excretion were of similar magnitude. Together these results indicate that diclofenac sodium might be used as a long-lasting and potent alternative to indomethacin and aspirin in experimental studies on the renal PG system in vivo.