1. The effect of A.I. on the structure of the pedigree British Friesian breed has been assessed from analyses of pedigrees of systematic samples of 221 registered females and 200 registered males born in 1960. Samples of 200 registered females and 200 registered males born in alternate years from 1946 to 1958 inclusive were used to estimate the trend in the proportion sired by A.I. bulls.

2. By 1960, A.I. had not affected the hierarchical structure of the breed and had not, therefore, narrowed its base. Current inbreeding was lower in artificially than in naturally sired stock but there was little difference between the two groups in the level of non-current inbreeding.

3. Over 50% of artificially sired stock were sired by bulls likely to have progeny test information. Of the naturally sired stock, only 17% of females and 29% of males were sired by such bulls.

4. 12·2% of genes of registered females born in 1960 derived from the 1950 importation from the Netherlands. At comparable points in time the contributions of earlier importations from the Netherlands were: 1914–39·9%; 1936–24·3%. The increased size of the pedigree breed and the development of progeny testing methods giving an earlier and more accurate assessment of the breeding worth of individual sires, probably contributed to the relatively small influence of the 1950 importation.