These drugs were chosen on the basis of their similarity of structure and the diversity of their pharmacological effects. Sch 12,679 is effective in reducing animal and human aggression, fenfluramine ia a wide ly used anorexigenic agent, PMA is purported to be a hallucinogen which is 5 times as potent as mescaline, and BL-3912 is claimed to possess certain of the actions of LSD but without its ability to produce visual hallucinations. With the exception of BL-3912, all of the cross tested drugs gave intermediate results, i.e., responding was fully appropriate for neither training condition. In contrast BL-3912 substituted completely for LSD. When animals were pretreated with BC-105, a serotonergic antagonist previously shown to be effective in blocking the stimulus effects cf LSD and mescaline, the LSD-like effects of Sch 12,679, fenflur ine, PMA, and BL-3912 were diminished.