Nucleation from the vapor phase on a foreign substrate with a low number of inhomogeneities is studied. Kinetic equations describing the 3D nucleation on active sites are solved numerically. It is shown that the cluster flux (i.e., the rate of; formation of nuclei of a given size) reaches some extremal value at certain time and then tends to zero as time increases; In such a sense, formation of nuclei is a fully nonstationary process, in contrast to the standard nucleation theory. The total number of nuclei formed bn a unit surface reaches for a sufficiently long time some stationary value which is close to the number of nucleation active sites.