For the Western Ontario and Michigan basins, which overlie the Grenville and Central geological provinces, temperature gradients in more than 30 boreholes and thermal conductivities of several hundred specimens from several formations have been determined. It is found that characteristic gradients and conductivities can be assigned to certain formations and to geological periods. There may also be a real variation (< 20%) of heat flow between some formations, which is qualitatively correlated with variations in heat producing elements.With the exception of one value of 1.3 μcal cm−2 s−1, which involved extrapolation of data to a great distance, all 'whole borehole' heat-flow values, uncorrected for glaciation effects, lie between 1.0 ± 0.2 μcal cm−2 s−1; a correction for the Wisconsin stage of the Pleistocene glaciation would increase these values by 0.3 ± 0.1 μcal cm−2 s−1.