  We present the results of the Infrared Space Observatory Short Wavelength
Spectrometer (ISO-SWS) observations of Jupiter related to ammonia. We focus on
two spectral regions; the first one (the 10-micron region), probes atmospheric
levels between 1 and 0.2 bar, while the second one (the 5-micron window),
sounds the atmosphere between 8 and 2 bars. The two spectral windows cannot be
fitted with the same ammonia vertical distribution. From the 10-micron region
we infer an ammonia distribution of about half the saturation profile above the
1-bar level, where the N/H ratio is roughly solar. A totally different picture
is derived from the 5-micron window, where we determine an upper limit of
3.7E-5 at 1 bar and find an increasing NH3 abundance at least down to 4 bar.
This profile is similar to the one measured by the Galileo probe (Folkner et
al. 1998). The discrepancy between the two spectral regions most likely arises
from the spatial heterogeneity of Jupiter, the 5-micron window sounding dry
areas unveiled by a locally thin cloud cover (the 5-micron hot spots), and the
10-micron region probing the mean jovian atmosphere above 1 bar. The 15NH3
mixing ratio is measured around 400 mbar from nu_2 band absorptions in the
10-micron region. We find the atmosphere of Jupiter highly depleted in 15N at
this pressure level (15N/14N)_{Jupiter}=(1.9^{+0.9}_{-1.))E-3, while
(15N/14N)_{earth}=3.68E-3. It is not clear whether this depletion reveals the
global jovian (15N/14N) ratio. Instead an isotopic fractionation process,
taking place during the ammonia cloud condensation, is indicated as a possible
mechanism. A fractionation coefficient alpha higher than 1.08 would explain the
observed isotopic ratio, but the lack of laboratory data does not allow us to
decide unambiguously on the origin of the observed low (15N/14N) ratio.
