The effect of renal failure upon the "in vitro" binding of midazolam, a new water-soluble short-acting benzodiazepine, has been studied in man. An increase of its free fraction (ranging from 2.52 to 5.17%) in serum from uremic patients was observed. A similar situation was originated in rabbits by administering uranyl nitrate (2 mg/Kg i.v.) and posterior hypnosis with midazolam. Uremic rabbits showed a marked increase in the free concentration of midazolam in serum (ranging from 8.9 to 13.7 micrograms/ml) and in midazolam brain levels (156.2 micrograms/g in cortex vs 84.5 micrograms/g in control animals). A positive correlation between brain and serum free concentration of midazolam was also observed. It is concluded that in renal patients more unbound drug is available to produce central nervous system effects, and a decrease in intravenous dose of midazolam could be recommended in this clinical situation.