A - G

M id-rapidity spectra and yields ofK (cid:0) and K + have been m easured for A u+ A u collisions at 4, 6,8,and 10.7 AG eV .The K (cid:0) yield increases faster with beam energy than for K + and hence the K (cid:0) /K + ratio increases with beam energy. This ratio is studied as a function ofboth p s and p s - p s th which allowsthedirectcom parison ofthekaon yieldswith respectto theproduction threshold in p+ p reactions.Forequal p s - p s th the m easured ratio K (cid:0) /K + = 0.2 atenergiesabove threshold in contrastto the K (cid:0) /K + ratio ofnearunity observed atenergies below threshold.The use ofthe K (cid:0) /K + ratio to testthe predicted changesofkaon propertiesin dense nuclearm atterisdiscussed.