The 40Ar/39Ar method using stepwise heating has been applied to 11 Precambrian mafic rocks in Minnesota and Ontario to test the technique on samples varying from extremely altered to essentially unaltered and to date some of the Lower and Middle Keweenawan igneous rocks. Four essentially unaltered samples give plateau ages which appear to be reliable and may date the time of intrusion. One strongly metamorphosed sample gives a plateau age which may date the time of metamorphism. The altered mafic rocks give patterns which are inconsistent, difficult to interpret, and have higher ages for the higher temperature fractions. Essentially unaltered samples may also give inconsistent patterns. For the Keweenawan igneous activity along the northwest shore of Lake Superior near the Minnesota–Ontario border and a Logan sill near Lake Nipigon, the plateau ages suggest that three Lower Keweenawan Logan intrusions occurred 1150 to 1170 m. y. ago, although it is possible that the sills near the Minnesota–Ontario border are older; that the Keweenawan igneous activity ended about 1135 m.y. ago, based on one late dike analysis; and that a basal Keweenawan basalt flow was metamorphosed and cooled enough to retain argon by 1095 m.y. ago.