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 i.Methods. The core was mapped in the J = 1−0 rotational lines of HCN, HNC, and HN 13 C. The column densities of H 13 CN, HN 13 C, H 15 NC and NH 3 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 NH 3 (1, 1) and (2, 2) inversion lines, is 12.1 ± 0.1 K. The HN 13 C/H 13 CN column density ratio is between 3 and 4, i.e. similar to values found in several other cold cores. The HN 13 C/H 15 NC column density ratio is ∼7. In case no 15 N fractionation occurs in HNC (as suggested by recent modelling results), the HNC/HN 13 C abundance ratio is in the range 30−40, which indicates a high degree of 13 C fractionation in HNC. Assuming no differential 13 C fractionation the HCN and HNC abundances are estimated to be ∼7 × 10 −10 and ∼2 × 10 −9 , respectively, the former being nearly two orders of magnitude smaller than that of NH 3 . 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: χ(NH 3 ) > χ(HC 3 N) > χ(HNC) > χ(HCN) > χ(N 2 H + ). 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.