The Marsh Chapel Experiment (a.k.a. "the Good Friday Experiment") was run by Walter N. Pahnke, a graduate student in theology at Harvard Divinity School, under the supervision of Timothy Leary and the Harvard Psilocybin Project. The goal was to see if in religiously predisposed subjects, psilocybin (the active principle in psilocybin mushrooms) would act as reliable entheogen. The experiment was conducted on Good Friday, 1962 at Boston University's Marsh Chapel. Prior to the Good Friday service, graduate degree divinity student volunteers from the Boston area were randomly divided into two groups. In a double-blind experiment, half of the students received psilocybin, while a control group received a large dose of niacin. Niacin produces clear physiological changes and thus was used as a psychoactive placebo. In at least some cases, those who received the niacin initially believed they had received the psychoactive drug.

However, the feeling of face flushing (turning red, feeling hot and tingly) produced by niacin subsided over the first hour or so. Meanwhile, the effects of the psilocybin intensified over the first few hours. Almost all of the members of the experimental group reported experiencing profound religious experiences, providing empirical support for the notion that psychedelic drugs can facilitate religious experiences.

In 2006, a more rigorously controlled version of this experiment was conducted at Johns Hopkins University by Roland R. Griffiths, Ph.D., yielding very similar results.