[en] The ratio of K- to K+-meson yields has been measured in the systems 96Ru+96Ru at 1.69 A GeV, 96Ru+96Zr at 1.69 A GeV, and 58Ni+58Ni at 1.93 A GeV incident beam kinetic energy. The yield ratio is observed to vary across the measured phase space. Relativistic transport-model calculations indicate that the data are best understood if in-medium modifications of the kaons are taken into account. (orig.)