The types of nodules erupted in kimberlite and believed to be of upper mantle origin are divided into five types: ( a ) peridotites and dunites, ( b ) garnet-pyroxenites, ( c ) eclogites and grospydites, ( d ) megacrysts, ( e ) amphibole-bearing and mica-rich types. The value of relative pressure-temperature estimates in determining the conditions of formation of the nodules is noted and such estimates are used to discuss the distribution of rock types and minerals in the upper mantle sampled by kimberlite. Emphasis is placed on garnet—lherzolites and it is found that there is little evidence for the restriction of deformed or undepleted varieties of garnet-lherzolite to particular depths. Phlogopite, apparently in equilibrium with the host garnet—lherzolite assemblage, appears to be restricted to rocks formed in the lower range of pressure and temperature estimates (less than about 1150 °C). Ilmenite-bearing and other megacrysts of relatively high Fe and Ti and low Cr types appear to have pressure-temperature estimates in the intermediate and higher part of the garnet—lherzolite range. The distribution of phlogopite and megacrysts may be related to melting processes. Present information does not suggest extensive changes in the pressure and temperature of formation of nodules as a consequence of diapiric or other activity associated with kimberlite genesis. The pressure-temperature gradients of garnet-lherzolites from individual pipes do not show any inflexions, but appear to be straight or gently curved. Allowing for the uncertainties in absolute pressure and temperature estimates, the pressure-temperature ranges and gradients of nodule suites are roughly in accord with geotherms based on geophysical data for an upper mantle with a convective circulation.