We have studied here for the first time the use of the log(H 0/[N 0]), log (H 0/[S 0]) diagnostic diagram for determining the metallicities and ionization parameters of the H 0 regions in the Local Group. An important advantage of using these lines is their proximity in wavelength, which both simplifies the observations and makes the de-reddening corrections much safer or even unnecessary. In order to study the behaviour of extragalactic H 0 regions in this diagram, we carried out a thorough bibliographical search comprising all H 0 regions in all those galaxies of the Local Group for which adequate information about the relative intensities of the four lines is available, and compared this observational data to theoretical grids based on MAPPINGS III (Sutherland and Dopita 1993) and CLOUDY 96b5 (Ferland 2002) photoionization codes. We found out that both the MAPPINGS and the CLOUDY models fit the observations well and our main result is that the studied diagram is a powerful tool for estimating metallicities in extragalactic H 0 regions.