The aim of the study was to investigate whether rumination time (RT) was related to estrus in dairy cattle. On 4 farms, cows were equipped with a microphone-based sensor system that allowed continuous recording (in blocks of 2 h) of RT. The analyzed data set consisted of 265 verified estrus cycles of 224 animals with artificial insemination leading to conception. The day of estrus (d 0) was defined as the day when estrus was identified either by measurement of physical activity or by visual observation. In estrous cows, RT was significantly reduced. With a duration of 355 min/d, the minimum RT was found on the day of estrus compared with the base level of 429 min/d during the reference period (the mean of 3 d before and 3 d after estrus). The average decrease in RT was 17% (74 min), ranging between -71 and +16% among animals. Herd and parity affected the RT decrease during estrus. Among the 4 analyzed herds, the RT decrease of cows in estrus ranged between 14% (60 min/d) and 24% (94 min/d). The decrease in RT was more pronounced in primiparous than in mature cows. In conclusion, RT is reduced on the day of estrus on average. The RT decrease during estrus was characterized by high variation among cows.