The spontaneous locomotor activity of rats was investigated in an open-field test. Apomorphine and d,1-amphetamine caused a dose dependent increase of locomotor activity. LSD, mescaline, and DMT in low doses were ineffective, when given alone, but caused a strong potentiation of the hypermotility induced by apomorphine and d,1-amphetamine. Cyproheptadine antagonized the potentiating effect of...