High spectral and time resolution observations have been carried out on the beta Cephei star 12 Lacertae. More than 750 spectra of high quality (signal-to-noise ratio above 200) were obtained on the SiIII lambda lambda 4553-4575 triplet lines. A frequency analysis performed on the radial velocity curves reveals the presence of 6 frequencies (of which 3 form an equidistant triplet), already detected in photometric data. A sine-fit of our data using all these frequencies show that over 40 years, periods and amplitudes did not change noticeably. The moment method is then used in an attempt to derive the pulsation parameters. Unfortunately, it turns out that the method as proposed by Mathias et al. (1994) is not accurate enough in case of a complicated pulsation structure, such as the one of 12 Lacertae. Because of this, we cannot deny, neither confirm previous mode identifications in this star. We mention, however, that a frequency analysis performed on the line parameters variations (residual intensity and full width at half maximum) are in favour of non-radial modes with a main sectoral one or a tesseral one with e - 1 = \m\. It seems that shock waves are not present in the pulsation.