The development of anticonvulsant tolerance with three benzodiazepines was assessed in mice using a slow intravenous infusion of pentylenetetrazol as the convulsive stimulus. Chlordiazepoxide (12.5 mg/kg b.d.) and midazolam (0.75 mg/kg b.d.) induced a slowly evolving tolerance over 15 days whereas nitrazepam (0.6 mg/kg b.d.) induced a very marked rapid tolerance which developed no further during 6 days treatment. Tolerance appeared to be incomplete with all three benzodiazepines. Possible explanations for the differences in tolerance profile are discussed and an alternative basis for the classification of benzodiazepines is suggested.