The genetic improvement of fertility in post-partum dairy cows is hampered by its age-limited and sex-limited expression since it can only be measured on adults and in the female. However improvement is further hampered by the low heritability of the traits that are commonly used as selection criteria (typically estimated ≤0.05), such as time to first insemination (T1), conception to first service (P1), number of inseminations per conception (NI) and days open (DO). Thus despite its undoubted economic importance in dairy cattle management, especially if the scheme is tied to an annual cycle, fertility does not receive a large weight in any aggregate selection index, since the weight in the index depends not only upon the economic importance but also upon how well genetic merit can be identified. Nevertheless significant opportunities may exist to improve fertility providing the appropriate traits are measured and recent evidence (Darwash et al. 1997) suggests that traits such as the interval to commencement of luteal activity postpartum (CLA) may have moderate heritability (0.21, s.e. 0.07) and be related to fertility (Whitmore et al. 1974). The present study was therefore undertaken with the objective of examining whether such a finding in the heritability of CLA is consistent with the published evidence on the low heritabilities of commonly measured criteria of fertility.