Existing information about the effect of inbreeding on production characteristics in dairy cattle leaves much to be desired. Sources are two-fold ; from experiments specially designed to get information on this and other aspects of inbreeding and also from the analysis of records made in closed herds. However, the experiments are few and in many of the second type of analysis the degree of inbreeding is not high. Furthermore, an examination of the detailed results in one breed, Holstein-Friesians in the U.S.A., shows little agreement in the results obtained. While agreeing in finding a decline in production as inbreeding progressed, the different investigators disagreed as to the magnitude of the effect. Even under ideal conditons, one should perhaps not expect too close an agreement because the genetic situation may well be quite different in the initial stock in the different experiments and genetic drift during inbreeding may further obscure the picture. In addition, if the inbreeding each generation is small, any selection that is practised may complicate the situation. Most of the programmes of deliberate inbreeding were not started simply to investigate the effects of the breeding system but rather to try to produce superior inbred stock.