K-Ar mineral and whole-rock ages for mafic dikes and one sill that intrude the early and middle Precambrian rocks of northeastern Minnesota and adjacent areas in Ontatio suggest times of intrusion at more than 2200, 1300, and 900 to 1100 m.y. ago. Samples which give K-Ar ages greater than 1300 m.y. show extensive alteration characteristic of low-grade metamorphism (prehnite-pumpellyite facies). For these samples, the whole-rock K-Ar ages have been lowered and suggest that the metamorphism occurred at about 1600 m.y. ago. The samples which give whole-rock K-Ar ages of 1300 m.y. or younger do not show the same type or extent of alteration, nor do the whole-rock K-Ar ages appear to have been appreciably lowered.