  Inclusive and semi-inclusive measurements are presented for antiproton
($\bar{p}$) production in proton-nucleus collisions at the AGS. The inclusive
yields per event increase strongly with increasing beam energy and decrease
slightly with increasing target mass. The $\bar{p}$ yield in 17.5 GeV/c p+Au
collisions decreases with grey track multiplicity, $N_g$, for $N_g>0$,
consistent with annihilation within the target nucleus. The relationship
between $N_g$ and the number of scatterings of the proton in the nucleus is
used to estimate the $\bar{p}$ annihilation cross section in the nuclear
medium. The resulting cross section is at least a factor of five smaller than
the free $\bar{p}-p$ annihilation cross section when assuming a small or
negligible formation time. Only with a long formation time can the data be
described with the free $\bar{p}-p$ annihilation cross section.
