  We present the first Chandra detection of a single X-ray cavity within the
interstellar medium of the small Fanaroff-Riley type I (FRI) radio galaxy NGC
5141. The X-ray surface brightness depression, located $\approx 4$ kpc away
from the galaxy center, is projected on the northern radio lobe, which is
completely contained within the galaxy. The thermal gas surrounding the cavity,
which extends to $\approx$ 20 kpc, has a bolometric X-ray luminosity (0.1 - 100
keV) of L${_X}\approx2\times10^{40}$ erg s$^{-1}$ and a temperature of
$kT\approx0.8$ keV. We calculated the total energy (E$_{cav} = 4PV \approx
10^{55}$ erg) required to inflate the cavity and its age ($t_{cav}\approx 9$
Myrs), assuming that it is filled with relativistic particles and rises
buoyantly. The inferred total cavity power is as low as
P$_{cav}=E_{cav}/t_{cav}\approx6\times10^{40}$ erg s$^{-1}$, which is the
lowest one among the radio-filled systems. Comparing $P_{cav}$ to the
bolometric X-ray luminosity (i.e., the cooling luminosity), we conclude that
NGC 5141's central active galactic nucleus can heat the interstellar medium and
balance its cooling luminosity, confirming that the $P_{cav}-L_{cool}$
relation, mainly tested on groups and clusters, also works for such a low-power
system.
