Dietary stimulation of insulin in post partum dairy cows has been found to enhance ovarian follicle development but to impair oocyte developmental competence. The objective of this study was to test the hypothesis that pregnancy rate would be improved by feeding a diet to stimulate higher insulin (H) until cows resumed ovarian cyclic activity after parturition, and then feeding a diet to lower insulin (L) during the mating period. Each diet was fed to 30 post partum dairy cows until their first rise in milk progesterone, when 15 cows in each group were transferred to the other diet (treatments HL and LH) and 15 cows in each group remained on their original diet (treatments HH and LL) until 120 days post partum. Treatments did not affect dry matter intake, milk yield and metabolisable energy balance. Plasma insulin concentration was elevated in cows fed on H compared with cows fed on L. Treatment did not affect days to first progesterone rise, first oestrus or first insemination. At 120 days post partum, 27% of cows on each of treatments HH, LL and LH were pregnant, but 60% of cows on treatment HL were pregnant (P=0.021). These findings support the concept that physiological relationships between insulin and the reproductive system vary according to stage of the reproductive cycle, and suggest that pregnancy rate can be enhanced by a two-diet strategy tailored to optimise responses before and after the first post partum ovulation.