12 K-Ar isotopic ages are reported from 7 dolerite dykes from the Kongsberg-Fiskum district, Norway. Ages obtained from the unaltered central parts of the dykes fall in the range 270-276 m. a. The marginal parts of the dykes and those samples selected as being representative of hydrothermally altered dolerites
give ages with a minimum of 237 m. a. The petrography and petrochemistry of the dolerites is discussed in relation to basaltic intrusions of Permian, Early Cambrian and Late Proterozoic age found in the southwestern part of the Baltic Shield. A summary of the vein deposits of the area is given, together with new
data on their spatia( and tempora( relationships to the dykes. The field relationships strongly suggest that the igneous activity is closely associated in time with vein formation. The ages of the dolerite dykes provide useful constraints on the age of the vein deposits, confirming ages previously obtained on wallrock alteration related to the veins.