We examined all of the galaxies in the Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7) to select those with a detected OIII [lambda]4363 emission line, which allows to derive electron temperatures in HII regions and element abundances by the directmethod. We selected two sub-samples of galaxies: one with detected WR features in their spectra, and the other with the nebular HeII [lambda]4686 emission line. We confirm the increase of the N/O abundance ratio with decrease of the equivalent width EW(H[beta]) of the H[beta] emission line. This result is explained by gradual nitrogen enrichment of the interstellar medium by ejecta from massive stars in the most recent starburst episode.