An arrangement for measuring rumination of an animal comprising: - a sensor comprising an accelerometer for repeatedly sensing an acceleration and configured to be attached to a portion of the animal, the movement of which being at least partly correlated to the movement of the jaws of the animal during rumination; and - a processing device operatively connected to the accelerometer to obtain the repeatedly sensed acceleration as an acceleration signal, characterized in that - the processing device is configured (i) to process the acceleration signal, thereby removing a DC component from the acceleration signal, (ii) to repeatedly determine a size of the processed acceleration signal in a selected frequency band, (iii) to determine a first threshold value, and at each time the size of the processed repeatedly sensed acceleration in the selected frequency band has been determined, (iv) to compare the last determined size of the processed acceleration signal in the selected frequency band and the first threshold value, and (v) to determine that the animal is ruminating if a rumination condition is met, wherein the rumination condition comprises that the last determined size of the processed acceleration signal in the selected frequency band is higher than the first threshold value, and - wherein the processing device is configured, at each time the size of the processed repeatedly sensed acceleration in the selected frequency band has been determined, to repeatedly determine a size of the processed acceleration signal outside the selected frequency band and to determine the first threshold value based on, e.g. to be equal to, the size of the processed acceleration signal outside the selected frequency band. - a sensor comprising an accelerometer for repeatedly sensing an acceleration and configured to be attached to a portion of the animal, the movement of which being at least partly correlated to the movement of the jaws of the animal during rumination; and - a processing device operatively connected to the accelerometer to obtain the repeatedly sensed acceleration as an acceleration signal, characterized in that - the processing device is configured (i) to process the acceleration signal, thereby removing a DC component from the acceleration signal, (ii) to repeatedly determine a size of the processed acceleration signal in a selected frequency band, (iii) to determine a first threshold value, and at each time the size of the processed repeatedly sensed acceleration in the selected frequency band has been determined, (iv) to compare the last determined size of the processed acceleration signal in the selected frequency band and the first threshold value, and (v) to determine that the animal is ruminating if a rumination condition is met, wherein the rumination condition comprises that the last determined size of the processed acceleration signal in the selected frequency band is higher than the first threshold value, and - wherein the processing device is configured, at each time the size of the processed repeatedly sensed acceleration in the selected frequency band has been determined, to repeatedly determine a size of the processed acceleration signal outside the selected frequency band and to determine the first threshold value based on, e.g. to be equal to, the size of the processed acceleration signal outside the selected frequency band.