The relation between the average magnetic fieldB, the angular velocity Ω, and the periodP of stellar activity cycles is studied. For the calculations we have used Leighton's (1969) model for the solar cycle with the additional assumption that the differential rotation and the cyclonic turbulence (Parker, 1955) (that is the ‘sunspot tilt’ or the ‘α-effect’) are both proportional to Ω. We then find thatB is roughly proportional to Ω and thatP decreases with increasing Ω. The period of the solar cycle increases therefore with the age of the Sun.