Research and discussion related to the behavioral and electroencephalographic effects of long-term use of marijuana or its constituents are reviewed. It is concluded that tolerance develops to the behavioral and electroencephalographic depressant aspects of the cannabis drugs but little attentuation develops to the stimulant effects. The rate of tolerance development is considerably influenced by learning factors such as the opportunity or necessity to make behavioral responses that counteract the decrements in performance resulting from the drug's influence. Evidence is presented indicating that the animal and human literature are in considerable agreement if socially acquired cues and expectancies and dosage/frequency parameters are taken into consideration.