The electron-phonon coupling is shown to depend on the carrier concentration n. As a result of this dependence, Tc reaches a maximum at some value of n. This maximum of Tc has been observed in the high Tc oxides. At a higher n, there is a superconductor-normal metal transition. The calculated carrier concentration which corresponds to Tmaxc is in agreement with experimental data. The dependence of the Sommerfeld constant λ on n is predicted to be non-monotonic. © 1990.