A diallel cross between two strains of rats from the same genetic source, that vary greatly in body size, was made in order to evaluate the differences in individual gain, feed consumption and feed efficiency (gain/feed) from 4 to 15 wk of age. Significant paternal, maternal and sex effects were evident for all weekly gains and feed consumption. These effects were not significant for feed efficiency. Offspring of Large Line (L) rats grew faster and consumed more feed than offspring of Small line (S) rats. Cross-bred mating types (LS and SL) were significantly different as SL rats were faster growing with greater feed consumption than LS rats indicating a significant maternal effect of L female. Heterosis for growth rate was positive and feed consumption negative; however, neither was significant. No significant line of sire, line of dam or heterosis effects on feed efficiency were seen. Analysis of this mating design indicates that increased gain was due to increased feed consumption rather than increased feed efficiency; and, any heterotic effects in this cross between extremes in body size, if present, were masked by maternal effects.