  We analyse the production and propagation of kaons and antikaons in Ni + Ni
reactions from 0.8--1.85 GeV/u within a coupled channel transport approach
including the channels $BB \to K^+YN, \pi B\to K^+Y, BB \to NN K \bar{K}, \pi
B\to N K\bar{K}, K^+B\to K^+B, \bar{K} B\to \bar{K}B, Y N\to \bar{K} NN, \pi
\pi\to K \bar{K}$ as well as $\pi Y\to \bar{K}N$ and $\bar{K} N\to \pi Y$ for
the antikaon absorption. Whereas the experimental $K^+$ spectra can be
reproduced without introducing any selfenergies for the mesons in Ni + Ni
collisions from 0.8 to 1.8 GeV/u, the $K^-$ yield is underestimated by a factor
of 5--7 at 1.66 and 1.85 GeV/u. However, introducing density dependent antikaon
masses as proposed by Kaplan and Nelson, the antikaon spectra can be reasonably
well described.
