The rare earth element content of rocks from the Sudbury Nickel Irruptive has been determined by neutron activation analysis. The Irruptive rocks are characterized by high absolute rare earth element contents and chondrite-normalized rare earth element fractionation patterns which show strong enrichment of light, relative to heavy rare earth elements. Correlative rock units from the north and south ranges have very similar chondrite-normalized rare earth element fractionation patterns. Micropegmatites and norites are characterized by negative and positive Eu anomalies, respectively, whereas border norites and oxide-rich gabbros lack prominent Eu anomalies.The Irruptive border norites are considered to have crystallized as a relatively closed system and their rare earth element fractionation patterns are regarded as representative of the parental Irruptive magma. The mirror-image Eu anomaly characteristics of the micropegmatite and norite and the likelihood that they arise from a nonanomalous parental magma indicate that these rocks are comagmatic and that they are related by gravitational fractional crystallization processes. The norites represent cumulates and the oxide-rich gabbros are transitional rocks with micropegmatites crystallizing from a residual liquid.Mass balance considerations based on Eu anomaly characteristics suggest that a micropegmatite to gabbro proportion of 1:3 should apply to the Irruptive as a whole rather than the observed outcrop proportions of 1:1 or greater. A funnel-shaped geometry for the Irruptive is inferred from these considerations.Compared to other continental basic rocks the Sudbury Irruptive has much more fractionated rare earth element patterns and generally higher total rare earth element contents. These properties together with the siliceous nature of the Irruptive rocks suggest that mixing of a silicic component of high La/Yb ratio with a normal basaltic melt may be responsible for the distinctive rare earth element characteristics of the Irruptive magma. Further, the Eu anomaly properties are most readily accounted for if mixing occurred prior to intrusion of the Irruptive magma to its present site, so that initial rare earth element characteristics are represented by the border norites and rare earth element properties of the main rock units developed by fractional crystallization in a substantially closed system.