Translational efficiency of wheat ribosomes was studied as a function of an in vivo temperature pretreatment of wheat seedlings. The ribosomes were isolated from 41 oř 36 oC-adapted and non-adapted (20 oQ wheat seedlings. The poly-U-dependent translational efficiency, measured as ^H phenylalanine incorporation into poly-Phe, was enhanced up to 3-fold in the heat-adapted ribosomes. The adaptive enhancement was due to the large ribosomal subunit, as demonstrated earlier by heterologous recombination of ribosomal subímits, obtained from the plants pretreated by different temperatures. According to this, the pattem of ribosomal proteins of the large subunit exhibited pronounced differences as a function of preadaptation temperature: one spot increased markedly in the protein staining intensity on the two-dimensional polyacrylamide gels, while another almost disappeared. Two minor protein spots disappeared at high preadaptation temperatures. An evaluation of the protein phosphorylation of ribosomal proteins yielded a decreased ^zp-iabel degree in čase of the smáli subunit of heat-adapted ribosomes. These results are considered to be an important molecular correlation to phenotypical temperature adaptatíon of in vivo protein synthesis in wheat, where the optimum temperature of ^‘♦C-leucine incorporation into the total protein fraction, as a measure of in vivo protein synthesis, shifts to higher grades with increasing preadaptation temperature of the wheat seedlings. Besides Triticum aestivum L. (spring wheat; cv. Kolibri), heat adaptatíon potentíals of T. dicoccoides (tetraploid), T. longissimum (2n), T. monococcum (2n), T speltoides (2«) and T. tauschii (2n) were investígated. The temperature coefficient p (apparent actívation energy) also underwent adaptive alteratíons, although these changes were not unidirectíonal. T. tauschii proved to be the species with the most pronounced adaptive potentíal in the high temperature range, surpassed only by the heat adaptability of 14 d-postanthesis caryopses: its optimum temperature of in vivo protein synthesis rose by more than 20 «€ after a 38 oC-preadaptation period (2 d).