  A three-dimensional weak coupling BCS model with an {\it anisotropic} pairing
interaction in momentum space is reported. It exhibits an anisotropic gap in
accord with recent experimental observations for high-$T_c$ oxides. The gap
ratio ${2 \Delta / k_B T_c }$ is calculated as function of the anisotropy ratio
of the electron pairing. It is found that the mean gap ratio decreases with
increasing anisotropy of the electron pairing, whereas the maximum gap ratio
increases. Assuming a unique strength of the effective electron interaction the
transition temperature turns out to be always larger than in the isotropic
case. The temperature dependence of the gap in the Cu-O plane and along the
c-axis of YBaCuO is presented.
