Research has shown that ovulation in beef females occurs at an average of 31±1 hours after the onset of estrus (range of 22 to 43 hours). If the onset of estrus could be determined with relative precision, the optimal time for insemination should be able to be determined. In this 3-year Oklahoma State Univ. study, a radiotelemetric system was used to detect when mounting occurred in Angus x Hereford beef heifers (n=161). Onset of estrus was defined as the first of two mounts received within 4 hours. Lutalyse® was used to induce estrus. The heifers were randomly allotted for AI at 1 to 4, 16 to 20, or 32 to 36 hours after the onset of estrus. Pregnancy was diagnosed 26 to 32 days after AI, using ultrasound. Pregnancy rate was not influenced significantly by time of AI, and averaged 62.3, 70.2, and 63.4% at 1 to 4, 16 to 20, and 32 to 36 hours, respectively. The authors stated, however, that because the number of heifers in each treatment was limited (n=49 to 57), additional observations may be necessary to conclusively determine if AI at 16 to 20 hours after the onset of estrus (11 to 15 hours prior to the expected time of ovulation) will increase pregnancy rate compared with AI at 1 to 4 and 32 to 36 hours after onset of estrus (Wettemann et al. 2004. Southern Section, ASAS, Abstract 76).