The effective interaction in nuclear matter is defined as the long-range part of the two-body potential, which, in the Born approximation, gives the single-particle potential energy for the average momentum in the Fermi sea. For the Brueckner-Gammel-Thaler potential the effective interaction has been calculated, first for the free-particle propagator and then for the nuclear spectrum. The result shows that in the first case the separation distance ${\ensuremath{\xi}}_{0}$ is constant over a wide range of densities and does not lead to saturation. The nuclear separation distance $\ensuremath{\xi}({k}_{F})$ changes quite rapidly with the Fermi momentum ${k}_{F}$; for low densities it is very close to ${\ensuremath{\xi}}_{0}$, while for higher densities it becomes very much larger. At the density corresponding to ${k}_{F}=1.5$ ${\mathrm{F}}^{\ensuremath{-}1}$, the long-range potential starts at $\ensuremath{\xi}=1.16$ F, and the rate of change of $\ensuremath{\xi}$ with ${k}_{F}$ is $(\frac{d\ensuremath{\xi}}{d{k}_{F}})=0.8$ ${\mathrm{F}}^{2}$. The minimum of the total energy per particle occurs at ${k}_{F}=1.35$ ${\mathrm{F}}^{\ensuremath{-}1}$ and is about -9 MeV. For ${k}_{F}=1.5$ ${\mathrm{F}}^{\ensuremath{-}1}$ the contributions of different partial waves are also calculated by a variational technique, and the results have been compared with previous calculations.