Causes of variation amongst cattle within a herd in their ability to initiate and maintain pregnancy are largely unknown. An experimental animal resource has recently been established to understand the biology of early reproductive performance. This paper summarises the results achieved during the establishment phase and from several experiments aimed at determining the physiological basis of the difference between sub-herds of contrasting pregnancy rates on Day 60. Each of 155 contemporary yearling heifers received 2 in vitro-produced embryos on 6 separate occasions during a 26-month period. Sixty days after transfer, pregnancy and twinning rates were determined ultrasonically, pregnancies terminated and the process repeated. The interval between successive transfers was greater than 100 days. Heifers were ranked on their aggregate pregnancy rate performance after 6 rounds of transfer, and the highest (High) and lowest (Low) 25 were retained. Differences in reproductive performance during the establishment phase of the herd are reported. In addition, several subsequent experiments examined ovarian follicle turnover and progesterone levels during an oestrous cycle, early embryo development after either AI or embryo transfer, and protein, interferon tau and ubiquitin-cross-reactive protein levels in uterine luminal flushings.Pregnancy rates were 7-folder higher in the High sub-herd (76 vs. 11%), with much of this difference apparent by Day 25. The proportion of heifers observed in standing oestrus prior to embryo transfer and the interval from the end of synchronisation treatment to the onset of oestrus were similar in the sub-herds. Oestrous cycle length, ovarian follicular dynamics and progesterone profiles during the oestrous cycle were also similar. More conceptuses had elongated by Day 14 in the High sub-herd (67 vs. 14%, P<0.05), which also tended to have a higher pregnancy rate after artificial insemination (52 vs. 29, P<0.1). Total protein in flushings from the uterus was similar in the sub-herds on Day 14 and Day 17. Conceptuses in the High sub-herd were longer on Day 17 following embryo transfer (6.5 vs. 4.8, P<0.05). Interferon-tau levels were higher in the High sub-herd (25.9 vs. 16.1, P<0.01), although ubiquitin cross-reactive protein levels were also higher in the High sub-herd, but this difference just failed to reach significance. We conclude that: 1. Most of the difference in sub-herd pregnancy rate occurs within 3 weeks of ET; 2. Ovarian factors are unlikely to contribute to the difference; 3. Major differences occur after blastocyst hatching and probably depend upon a differing endometrial environment before Day 14; 4. Differences in the ability of the uterine milieu to stimulate the expression of interferon-tau may be responsible for the differences in pregnancy rate; 5. The two sub-herds are a unique experimental resource for understanding early pregnancy in cattle following either AI or ET.