  Mid-rapidity spectra and yields of K$^-$ and K$^+$ have been measured for
Au+Au collisions at 4, 6, 8, and 10.7 AGeV. The K$^-$ yield increases faster
with beam energy than for K$^+$ and hence the K$^-$/K$^+$ ratio increases with
beam energy. This ratio is studied as a function of both $\sqrt{s}$ and
$\sqrt{s}$-$\sqrt{s_{th}}$ which allows the direct comparison of the kaon
yields with respect to the production threshold in p+p reactions. For equal
$\sqrt{s}$ - $\sqrt{s_{th}}$ the measured ratio K$^-$/K$^+$=0.2 at energies
above threshold in contrast to the K$^-$/K$^+$ ratio of near unity observed at
energies below threshold. The use of the K$^-$/K$^+$ ratio to test the
predicted changes of kaon properties in dense nuclear matter is discussed.
