Samples were taken from diabase dykes and related contact rocks and from a differentiated intrusive body of the District of Mackenzie in the Yellowknife–Prosperous Lake area, the Lac de Gras area, the Point Lake area, and the Tree River – Coronation Gulf area. In addition, several samples were taken from basic intrusions of Ontario, and some of the results obtained from these are included.Radiometric dating by the potassium–argon method indicated at least four periods of diabase dyke intrusion in the Slave Province of the Precambrian shield about 2 200–2 400 m.y. ago, 1 900–2 100 m.y. ago, 1 100–1 200 m.y. ago, and 600–700 m.y. ago. Scatter in the radiometric dates prevents recognition of other possible events by use of the whole-rock method, where the age of intrusion is older than Middle Proterozoic.Chemical and petrologic studies were carried out in conjunction with the radiometric dating. Precise determination of potassium proved difficult; in most cases, a best value for each sample was selected from the results of three independent determinations. There is some suggestion of an increase in the potassium content of diabase dykes in the Canadian shield throughout Precambrian time. Alkali–silica ratios show that all of the diabase dykes studied belong to the world-wide tholeiitic magma type.