Grenville rocks from a 2500 km2 area centered on Otter Lake, Quebec (some 75 km northwest of Ottawa) are in the uppermost amphibolite to lower granulite facies; orthopyroxene occurs occasionally in both metabasic and charnockitic rocks. The temperature of metapmorphism was approximately 675 °C, based upon oxide, feldspar, and garnet–clinopyroxene thermometry. Little thermal gradient could be detected across the area. Carbonate thermometry, using reintegrated calcite compositions, yielded lower temperatures of 600 °C (maximum), while garnet–biotite and other Kd thermometers yielded scattered and for the most part unreasonable results. Metamorphic pressure, calculated from the reaction anorthite = grossular + sillimanite + quartz, was 5.0 ± 0.5 kbar(500 ± 50 MPa). Similar calculations based upon the reactions garnet + quartz = anorthite + orthopyroxene and garnet + quartz = anorthite + clinopyroxene yielded pressures of 5.5–7.0 kbar (550–700 MPa). Pressure calculations based upon assemblages of cordierite–garnet–sillimanite–quartz were less precise, but agreed with the outer estimates. Similar metamorphic temperatures and slightly lower pressures have been estimated for the Adirondack Lowlands of New York. In the Morin Highlands, 100 km east of Otter Lake, and in the Adirondack Highlands, 100 km east of the Adirondack Lowlands, temperatures of metamorphism (700–800 °C) and pressures of metamorphism (6–9 kbar (600–900 MPa)) are both higher. Thus it appears that over an approximate 300 km north–south direction nearly constant metamorphic conditions prevailed at Grenville time. In the east–west direction significant variations in metamorphic grade are recorded; both temperature and pressure markedly increase to the east.