  Aims. The purpose of this study is to investigate the distributions of the
isomeric molecules HCN and HNC and estimate their abundance ratio in the
protostellar core Cha-MMS1 located in Chamaeleon {\sc i}.
  Methods. The core was mapped in the J=1-0 rotational lines of HCN, HNC, and
HN13C. The column densities of H13CN, HN13C, H15NC and NH3 were estimated
towards the centre of the core.
  Results. The core is well delineated in all three maps. The kinetic
temperature in the core, derived from the NH3 (1,1) and (2,2) inversion lines,
is 12.1+/-0.1 K. The HN13C/H13CN column density ratio is between 3 and 4, i.e.
similar to values found in several other cold cores. The HN13C/H15NC column
density ratio is about 7. In case no 15N fractionation occurs in HNC (as
suggested by recent modelling results), the HNC/HN13C abundance ratio is in the
range 30-40, which indicates a high degree of 13C fractionation in HNC.
Assuming no differential 13C fractionation the HCN and HNC abundances are
estimated to be about 7E-10 and about 2E-9, respectively, the former being
nearly two orders of magnitude smaller than that of NH3. Using also previously
determined column densities in Cha-MMS1, we can put the most commonly observed
nitrogenous molecules in the following order according to their fractional
abundances: X(NH3) > X(HC3N) > X(HNC) > X(HCN) > X(N2H+).
  Conclusions. The relationships between molecular abundances suggest that
Cha-MMS1 represents an evolved chemical stage, experiencing at present the
'late-time' cyanopolyyne peak. The possibility that the relatively high HNC/HCN
ratio derived here is only valid for the $^{13}$C isotopic substitutes cannot
be excluded on the basis of the present and other available data.
