Time evolution of the cosmological ''constant''

We discuss cosmologies where the cosmological constant λ depends on time. The requirements of realistic cosmology impose restrictions on the functional dependence of λ. on the Hubble parameter H. We show that for a wide class of functions with λ. of the order of H3 the system of field equations leads to a stable fix-point behaviour with λ naturally very small today. The age of the universe, critical matter density and deceleration parameter may be modified. (orig.)