The Geochronological laboratory in Stockholm has since 1966, been carrying out Rb-Sr dating on rocks from northern Sweden. The results obtained imply a general pattern for the development of the Svecofennian orogenic zone characterized by the following three periods: 1. A period of deposition from 2100? to 1900 m.y. during which supracrustal rocks of eastern, Karelian facies were formed in the northeastern part and of western, svionian facies in the southwestern part of the area. 2. A period of intrusion- and deformation between 1900 and 1775 m.y. during which the main tectonic development occurred and most granites were formed. 3. A period of deposition and intrusion from 1775 to 1540 m.y. when a widespread volcanic activity occurred in the western and southwestern part of the orogenic zone combined with the intrusion of intermediate and acid rocks, some of which may be of anatectic origin. The deformation was slight during this generally postdeformative period but in some areas the supracrustal rocks have been intensively folded during the emplacement of the intrusive rocks of this period.