PGF2ª analogous (dinoprost) was administered (30 mg i.m.) to 14 postpartum cows on days 7 (PGD7) and 13 (PGD13) ofthe estrous cycle (day of heat =O) in arder to induce growth and ovulation of the dominant follicles during the first and second follicular waves, as well as to study the effect on heat onset, timing and pregnancy rate. Immediately after PGF2ª administration, a "heat mount detector" (HMD) was placed at the base of the tail of each animal. A control group of 14 cows, which did not receive the PGF2ª, provided comparative pre-treatment and pregnancy rate data. Visual heat detection was conducted at 3 hour intervals between 6:00 and 00:00; and recorded by HMD between 00:00 and 6:00 am. Heat incidence was greater during daylight hours (81 % prior to treatment p<0.05 and 88% after p<0.01) than during the night. Although heat frequency was similar for PGD7 (85.7%) and PGD13 (78.6%), it occurred earlier (p<0.01) and with greater regularity in PGD7 (53.2 ± 6.9 hours) than in PGD13 (74.6 ± 22.9 hours). These results were not affected by calving season. Half the of cows (n=7) in each group failed to retum to heat by day 25 post A.I., indicating possible gestation, but pregnancy rate subsequently diminished dueto embryo mortality, most notably in PGDl 3. These results indicate that PGF2ª induced regression of the corpus luteum on day 7 of the estrous cycle and subsequent stimulation (erhile rising progesterone Jevels) of growth and ovulation of the dominant follicle of the first foUicular · wave, produced earlies onset and reduced variability of heat between the cows tested.