Background and Rationale: Mescaline is a classic psychedelic. Despite its long history in human use, modern data about dose-dependent acute subjective effects are lacking. Research Question and Hypothesis: The study explored the role of the 5-HT2A receptor in mescaline-induced altered states of consciousness using escalating doses of mescaline and the 5-HT2A receptor blocker ketanserin co-administered with 800 mg mescaline in healthy volunteers. Methods and Analysis: We used a double-blind, placebo-controlled, 6-period cross-over design with six treatment conditions. Sixteen subjects (8 women) were included and received placebo, mescaline (100, 200, 400, and 800 mg), and 800 mg mescaline together with the 5- HT2A receptor antagonist ketanserin (40 mg). The order was randomized and counterbalanced with wash-out periods of at least 14 days between the test days. We assessed acute psychological effects with a visual analogue scale (VAS) and plasma concentration up to 30 h. Mystical type experiences were characterized with the mystical experience questionnaire (MEQ) at the end of the study day. Main Findings: Mescaline induced subjective effects changed dose dependently and were attenuated by ketanserin. Maximal mescaline plasma concentrations were reached after approximately 2 hours. Time to peak subjective effects (“any drug effect”) was reached after approximately 3 hours and effect duration increased dose-dependent from 6.4-14 h. Plasma half-life was around 3.7 h. Mystical type experiences on the MEQ increased for doses >100 mg mescaline significantly compared with placebo. Conclusions: Alterations of consciousness induced by mescaline are mainly mediated via 5- HT2A receptor and increased dose-dependently. Pharmacokinetics were dose-proportional.