The effect of regular full-sib and double first cousin mating on body weight and rate of gain over six generations of Japanese quail was determined. 2. The depression of characters was more pronounced in the full-sib than in the double first cousin mating system, the relative depression due to parental inbreeding and offspring inbreeding depending on the magnitude of parental effect and individual's own genotypic effect on the character. 3. The body weight and rate of gain taken at an early age showed greater depression due to parental inbreeding whereas that taken near sexual maturity was depressed mainly due to individual's own inbreeding.
