  Understanding the depletion of heavy elements is a fundamental step towards
determining the structure of pre-protostellar cores just prior to collapse. We
study the dependence of the NO abundance on position in the pre-protostellar
cores L1544 and L183. We observed the 150 GHz and 250~GHz transitions of NO and
the 93 GHz transitions of \NTHP \ towards L1544 and L183 using the IRAM 30 m
telescope. We compare the variation of the NO column density with position in
these objects with the H column density derived from dust emission
measurements. We find that NO behaves differently from \NTHP \ and appears to
be partially depleted in the high density core of L1544. Other
oxygen-containing compounds are also likely to be partially depleted in
dense-core nuclei. The principal conclusions are that: the prestellar core
L1544 is likely to be 'carbon-rich'; the nitrogen chemistry did not reach
equilibrium prior to gravitational collapse, and nitrogen is initially (at
densities of the order of $10^4$~cm$^{-3}$) mainly in atomic form; the grain
sticking probabilities of atomic C, N and, probably, O are significantly
smaller than unity.
