Effects of 5 mg/kg diazepam given once a week for several weeks to drug-naive and drug-experienced monkeys were studied on a spatial delayed-alternation task. In both groups response latency was increased and the number of correct responses was reduced. The effects were consistent over weeks in the experienced monkeys, but in the naive monkeys there were greater initial effects on response latency and on repeat errors, and these lessened over 2–3 weeks. Thereafter, effects resembled those observed in the experienced group. Results are discussed in terms of the possible factors involved in the development and persistence of tolerance to benzodiazepines.