The incidence of birth has been determined for each hour of the day for all births in Norway in 1968-1977 of fetuses of 16 weeks of gestation or older, with resident mothers. The 24-hour incidence variations of births (A) with spontaneous onset and parturition, (B) with spontaneous onset, but delivery intervention, (C) with induced onset, but spontaneous birth, and (D) with induced onset and delivery intervention, are all different. It is shown that the curve for the hourly incidence of birth category A coincides very well with previous results of other workers. When multiple births are excluded and category A is split into first and later births in Northern and Southern Norway, dissimilarities arise between the respective 24-hour incidence curves. The results indicate that the 24-hour birth incidence variation has an underlying endogenous, circadian rhythmicity - possibly synchronized by the sun. The 24-hour rhythmicities of birth categories B, C and D seem to be purely exogenous - reflecting the working activity rhythms of hospital obstetricians and midwives.