  We investigated the type of the carriers doped in the CuO$_2$-planes of
non-superconducting Pr$_{1+x}$Ba$_{2-x}$Cu$_3$O$_7$ single crystals by nuclear
magnetic resonance of Pr and chain-site Cu. The different spectra in the solid
solution system show that the holes in Pr-rich crystals are localized in states
similar to the ones in stoichiometric samples, that is in the
$4f^2-2p\pi$-hybridization band proposed by Fehrenbacher and Rice. In contrast,
the spectra of crystals prepared under conditions favoring Ba-rich phases
indicate the presence of holes in Zhang-Rice singlets in the CuO$_2$-plane. Our
results strongly support the model that superconductivity reported recently for
Pr123-crystals is due to Ba-rich regions in the inhomogeneous samples.
