The objective of this study was to determine if there is an association between level of milk production and duration of estrus as determined by standing activity recorded by a radiotelemetry system. Holstein cows (n = 267; 50 DIM) were fitted with a transmitter that allowed continuous recording of standing activity. Cows were housed in a free-stall barn and milked twice daily. Ovulation was confirmed for all estruses (n = 380). Average milk production for the 10 days before the day of estrus was used to classify cows as lower (< 39.5 kg/day) or higher (>/= 39.5 kg/day) producers at the time of estrous expression. Follicle size and serum estradiol (E(2)) concentrations were determined in a subset of cows (n = 71) on the day of estrus. Duration (6.2 +/- 0.5 h versus 10.9 +/- 0.7 h; P < 0.0001), standing events (6.3 +/- 0.4 versus 8.8 +/- 0.6; P = 0.001), and standing time (21.7 +/- 1.3 s versus 28.2 +/- 1.9 s; P = 0.007) were shorter for estruses from higher (46.4 +/- 0.4 kg/day; n = 146) than lower producers (33.5 +/- 0.3 kg/day; n = 177). Milk production was correlated with the duration of estrus (r = -0.51; P < 0.0001; n = 323). Higher producers had lower E(2) concentrations than lower producers (6.8 +/- 0.5; n = 31 versus 8.6 +/- 0.5 pg/ml; n = 40; P = 0.01) in spite of larger pre-ovulatory follicle diameter (18.6 +/- 0.3; n = 31 versus 17.4 +/- 0.2 mm; n = 40; P 0.004). Interestingly, E(2) concentrations were not correlated with diameter of the pre-ovulatory follicle (r = -0.17; P = 0.15) but milk production was correlated with both E(2) concentrations (r = -0.57; P < 0.0001) and diameter of the pre-ovulatory follicle (r = 0.45; P < 0.0001). Thus, high milk production decreases duration of estrus probably due to decreased circulating concentrations of E(2).