Data were from 44,707 cows and first-calf heifers in a commercial artificial insemination program. The percentage of heifers first seen in estrus in the morning was 73.2, not different from 72.6 for cows. Unfrozen semen was used for insemination. The 150- to 180-day percent nonreturns for cows inseminated before noon the same day (67.1) was not different from those inseminated between noon and 1800h (69.9) and after 1800h (68.9) the same day. However, insemination the following morning was too late for best results (62.7%). Cows first seen in estrus in the evening should be inseminated by noon the next day. The nonreturn rate for cows inseminated after 1400h the next day resulted in 63.8% 150- to 180-day nonreturns. A single mid-morning insemination for all cows first seen in estrus the night before or the same morning should give near maximum conception.