The present paper describes a study in which the association between the shape of the lactation curve based ondaily milk yield recordings and the registration of the first post partum heat as detected by an automated heat detectionsystem was examined. In total, 689 animals from 2 herds were included in the study. On both farms, the daily milkproduction was automatically registered by the management system. This data was used to determine in which weekpost partum the peak lactation occurred. Then the shape of the lactation curve between the first week and the peakweek was calculated, which served as a good reflection of the rapidity with which the milk production starts up.Both herds were equipped with similar pedometers to detect cows in heat in a standardized and reasonably accurateway. The first heat was registered on average 52 days post partum. Factors significantly associated with the intervalbetween parturition and first estrus were parity and maximum milk yield. Older cows and cows with a higher peakproduction were detected in heat significantly later. The rapidity with which the lactation starts up (expressed as theshape of the lactation curve) and the calving season were not significantly associated with the interval betweenparturition and first estrus.