Trained rats given LSD daily developed tolerance to the behavioral effects of LSD, as evidenced by the time required to climb a rope. The development of tolerance was variable when LSD was given at intervals of 48 hours. No tolerance was observed when LSD was given at intervals of 72 hours. . Tolerance to the behavioral effects of mescaline did not develop as rapidly as that to LSD. . Tolerance to the bradycardic effect of LSD in rats was not observed.
