The Okanagan and Similkameen plutonic complexes west of the Okanagan Valley of south-central British Columbia yield K–Ar dates that range from 185 to 133 m.y. East of the Okanagan Valley Shuswap gneisses into which the plutonics intrude, and which may be as old as pre-midCarboniferous in age yield K–Ar dates between 59.9 and 47.4 m.y. This abrupt change, which approximately coincides with the Okanagan Valley, is a consequence of an intense thermal event in the early Tertiary which has reset K–Ar dates in the gneisses at shallow depths. Comparison of K–Ar, sphene and apatite fission track dates demonstrates that the heating affected the plutons west of the Okanagan Valley and that cooling of the Shuswap gneisses occurred at a rate in excess of 25 °C. per million years. The scatter observed in the older K–Ar dates of the plutonic complexes could be caused by post-emplacement heating with variable partial argon loss rather than by separate magmatic events. Thus, only the oldesl K–Ar dates obtained from the plutons may be significant as minimum ages for emplacement.