Altitudinal variations in the content of main bioactive secondary metabolites namely, hypericin, pseudohypericin, hyperforin, the chlorogenic, neochlorogenic, caffeic and 2,4-dihydroxybenzoic acids, isoquercitrin, quercitrin, avicularin, hyperoside, rutin, biapigenin, (+)-catechin, (-)-epicatechin and in the composition of essential oil (EO) among Hypericum perforatum populations from two growing sites of Northern Turkey were investigated in detail for the first time. Aerial parts of 30 individuals at full flowering were collected at five different altitudes (391, 631, 1318, 1586 and 1733 m) of "Anzer" upland and six different altitudes (1311, 1447, 1968, 2068, 20169 and 2210 m) of "Cimil" upland in territory of Rize province, Turkey. Air dried plant samples were assayed for polar ingredients by high-performance liquid chromatography (HPLC) and for EO components by solid-phase microextraction (SPME) supported gas chromatography-mass spectrometry (GCMS) analyses. All the tested polar phytochemicals were detected at various levels depending on altitude in both uplands and the accumulation level of each compound showed a positive consistent and significant response to altitudinal gradient. A considerable diversity in quality and quantity of major EO components was also observed and 2,2,6-trimethyl-cyclohexanone, caryophyllene oxide, tricycloheptane, hexanoic acid, tetradecanol, 2-methyl-dodecane, tetradecane, a-amorphene, eicosene, beta-cedrene, piperitone, spathulenol, beta-caryophyllene, d-cadinene and linalool were identified as major EO components depending on altitude. The present results suggest that altitude of plant habitat has an ultimate effect on secondary metabolism of Hypericum perforatum and indicate the investigated populations as new chemotypes in terms of their EO composition. Such kind of findings could be useful to characterize the unique strains with desired chemical composition and can provide new viewpoints for investigations on this valuable medicinal plant. (c) 2020 SAAB. Published by Elsevier B.V. All rights reserved.