Pituitary tissue contains phenol sulfotransferase (PST), the enzyme that catalyzes the sulfate conjugation of monoamine neurotransmitters. We carried out these studies with pituitaries obtained 21.3 ± 3.0 h postmortem (mean ± SEM; n = 21) to determine whether the biochemical properties and variations in levels of human pituitary PST activities were similar to those of PST in platelets from control subjects. PST in the human platelet has been studied thoroughly because of the possibility that platelet PST might reflect levels of PST activity in other tissues such as the pituitary and brain. Our results demonstrated 2 forms of the pituitary enzyme that were similar to the thermostable (TS) and thermolabile (TL) forms of platelet PST with regard to assay conditions, pH optima, Km values for multiple substrates, responses to 2,6-dichloro-4-nitrophenol (DCNP), and thermal stability properties. Pituitary samples also were obtained at autopsy 6.3 ± 0.33 h (mean ± SEM; n = 3) after death to determine the effects of storage at 4 °C on PST activities. After storage for 6–18 h, 83–99.6% of the TS PST activity remained and 44–66.9% of the TL PST activity remained. Pituitary TS PST activity in samples obtained within 12.1 ± 3.25 h after death was 121.0 ± 49.1 units/mg protein (mean ± SEM; n = 7) with a range from 9.7 to 367.6. TL PST activity was 35.6 ± 11.6 units/mg protein (mean ± SEM; n = 6) with a range from 6.1 to 80.7. Wide variations of both enzyme activities were also present in 3 pituitary tumor samples. Therefore, at least 2 forms of PST were present in the human pituitary, and the pituitary enzymes were biochemically similar to platelet TS and TL PST activities. It will now be feasible to determine whether variations in the levels of platelet PST activities might be useful measures of normal and tumor pituitary