Norbaeocystin, also known as 4-phosphoryloxytryptamine (4-PO-T), is a psilocybin mushroom alkaloid and analog of psilocybin. It is found as a minor compound in most psilocybin mushrooms together with psilocin, psilocybin, aeruginascin, and baeocystin, from which it is a derivative. Norbaeocystin is an N-demethylated derivative of baeocystin (itself an N-demethylated derivative of psilocybin), and a phosphorylated derivative of 4-hydroxytryptamine (4-HT). The latter is notable as a positional isomer of serotonin, which is 5-hydroxytryptamine. Norbaeocystin is thought to be a prodrug of 4-HT, analogously to how psilocybin is a prodrug of psilocin and baeocystin is thought to be a prodrug of norpsilocin. 4-HT is a potent and centrally penetrant serotonin 5-HT2A receptor agonist and also interacts with other serotonin receptors. In spite of this however, 4-HT and norbaeocystin do not produce the head-twitch response, a behavioral proxy of psychedelic effects, in animals, and hence are putatively non-hallucinogenic. The reasons for this are unknown, but may be due to β-arrestin2-preferring biased agonism of the serotonin 5-HT2A receptor.