The lines of the rare isotopes (H-13)CN, H(C-15)N, H(N-13)C, and (H-15)NC, which are sufficiently weak to be little affected by saturation, are used to measure the relative abundance of the isomers HCN and HNC in four giant molecular clouds. The low values of the HNC/HCN abundance ratio, which range from 0.015 to 0.40, are (1) similar to those found in warm sources from the common isotopes by Wootten et al. (1978), (2) not consistent with HNC and HCN production solely from various sources of the precursor ion H2CN(+), and (3) indicate that another source of HCN is required in warm, giant molecular clouds. It is suggested that neutral atom-molecule reactions may be an important contribution to the HCN production rate in such regions.