We have carried out optical spectroscopic observations at intermediate spectral resolution of the massive high redshift radio galaxy 0902+34 at z ≈ 3.39. This source was first identified by Lilly (1988) (from hereafter L88). The study of high redshift radio galaxies is interesting to analyze the physical conditions of the early universe and the galaxy evolution at cosmological redshifts. It has been claimed that some of these systems may be protogalaxies in the process of formation. Indications for this are the flat spectrum and the absence of the 4000 Å break, features which have already been observed in many cases. In particular, observations in the spectral range from V to K suggest that 0902+34 is a young galaxy (Eisenhardt and Dickinson 1992). Recent radio observations of the 21 cm line of neutral hydrogen have detected (Uson et al. 1991) an absorption against the radio continuum source. This absorption could also leave a track in the optical, redwards the Lyα line. Our observations were carried out with the ISIS spectrograph at the 4.2 m William Herschel Telescope (seeing ≈ 1.2–1.6 arcsec). A spectral dispersion of 0.78 Å/pixel (blue arm) and 1.38 Å/pixel (red arm) was obtained. Å long slit of width 3′ was used providing a spectral resolution of ≈ 5.4 Å in the blue arm and of ≈ 9.5 Å in the red one. Both resolutions are a clear improvement over that achieved by L88 of 20 Å, allowing us to resolve the Lyα line (and its possible structure) and any other possible strong features appearing in the spectral range observed (e. g., C iv λ1549, He ii λ1640, …). Six different observations of 2700 s of the radio galaxy 0902+34 were carried out. The slit was rotated to coincide with the parallactic angle at the beginning of each exposure. This will allow us to map spectroscopically different regions of the galaxy (for more details see Martín-Mirones et al. 1994).