I. Some aspects were studied of climax (hatching) behaviour in the domestic fowl. 2. The behavioural unit of climax behaviour has two functions: (a) cracking part of the eggshell, and (b) causing the animal to turn some degrees inside the shell. The concomittant movements are described with the aid of film analysis (Section I). 3. On the basis of their causal factors two aspects of climax behaviour can be distinguished: the stereotyped motility pattern of the behavioural unit and the rhytmical occurrence of the behaviour (the movements are repeated every 10 to 30 seconds). The stereotyped pattern only occurs when the freedom of movement of the animal is very restricted, whereas the rhythm already occurs if the animal is not permitted to leave its place while, in addition, its possibilities for postural adjustment are limited (Section 2). 4. After the onset of climax, a constant external stimulation (unbreakable egg and constant ambient temperature) will lead to response waning as a result of habituation, which is a function of (a) time, and (b) the degree of departure from the optimal incubation temperature (Section 3). 5. The degrees of stereotypy of the duration of a unit, proved to be higher than that of the duration of the interval between two consecutive units. 6. On the basis of behavioural and polygraphic criteria and with the aid of drugs it is shown that climax behaviour occurs while the animal is awake or drowsy, but - in contrast to what was thought - not during paradoxical sleep. Paradoxical sleep only occurs when - as a result of the earlier mentioned habituation - the tendency for climax behaviour has become very small (Section 4). 7. The interpretation of the findings is depicted in a relation diagram (Section 5).