Fertility indices, such as non-return rate and number of services per conception, have worsened significantly over the past few years. Because oestrus detection is one of the major factors influencing these indices, this study was performed to determine whether, how long, and at what time dairy cows show signs of oestrus in their behaviour. Two high producing herds with about 40 open cows were observed for 30 minutes every 2 hours for 6 weeks. The various signs of oestrus were recorded. The milk progesterone concentration was monitored daily to detect ovulation. A scale (Table 2) was composed according to the frequency of these symptoms during and between oestrous periods. This scale was used in order to determine which cows were in oestrus. Each oestrous symptom was given a score and the scores were summed for a 24-hour period. If a score of > 100 points was reached within a 24-h period, the cow was considered to be in oestrus. As used, the oestrus detection scale proved to be very convenient. Because standing oestrus was observed in only 37% of the oestri, this criterion had a very low sensitivity for oestrus detection. With the use of this scale and 12 observations per day, we reached a detection rate of 100%. If a more practical observation scheme would have been used (e.g. observing the herd twice daily, for 30 min) lower threshold values would have been obtained. This observation scheme resulted in a detection rate of 74% with no incorrect assessments.