Summary Twelve rocks for which calculated isotopic ages are mutually consistent and for which there is some biostratigraphic control have been used in the construction of a Permian time-scale. On the evidence of these rocks it is suggested that the Permian period lasted from about 280 m.y to about 225 m.y. ago, with the Lower Permian–Upper Permian transition at about 240 m.y. Ages are also suggested for the remaining intra-Permian stage boundaries.