Chronic administration of 5-methoxy-N,N-dimethyltryptamine (5-MeODMT, 2 mg/kg i.p., every 30 min for 4 h) produced a dramatic tolerance to the behavioral effects of the drug in rats. The ED50 for the syndrome-inducing effects of the drug was increased from 1.3 to 2.4 mg/kg, and the mean duration of the syndrome was decreased from 14.9 to 1.2 min after this treatment. This tolerance effect...