This chapter focuses on self-experimentation with hallucinogens within the mental health sciences. Emphasis is given to the use of LSD by mental health professionals in the former Czechoslovakia. The text is based on four main data sources: (1) interviews with mental health professionals who experimented with LSD on themselves in the former Czechoslovakia; (2) scientific articles that were published in the time LSD research was legal in the former Czechoslovakia; (3) unpublished materials from that era; and (4) books by those mental health professionals who engaged in self-experimentation with hallucinogens during that period. The chapter starts with an introduction to self-experiments with mescaline, which preceded those with LSD. The chapter continues with a description of early LSD work in the former Czechoslovakia, which began in 1952 and continued until 1974. We identified four major mental health centers where LSD self-experimentation was conducted. We characterize the context, theoretical orientations, the results of their research, and the impact that it had. Reflections of individuals directly involved in this work conclude the chapter.