-EJB 94 0574/2 80 S ribosomes from a number of higher eukaryotic organisms are able to hydrolyse ATP and GTP without the addition of soluble protein factors. ATPase seems to be an intrinsic activity of the ribosome, as indicated by the findings that ATPase activity is not diminished upon dissociation of ribosomes and reassociation of subunits, by washing with 0.66 M (KC1 + NH,Cl) or 0.6 M LiCl treatment and ethanol precipitation; 1.5 M LiCl treatment removes only 40% ATPase activity. 80 S ribosomes are able to bind a variety of NTPs, NDPs and NTP analogues, with a preference for ATP. Effective inhibitors of the ribosomal ATPase are ammonium metavanadate and alcaloid emetine. The ATPase activity is present on both ribosomal subunits, which may reflect the existence of two catalytical sites for ATP on the 80 S ribosome. Ribosomal ATPase is stimulated by the occupancy of the A site, in particular with charged tRNA. The ATPase inhibitor adenylylimidodiphosphate almost completely prevents elongation-factor(EF)-1-dependent binding of Phe-tRNAPh" to the A site. The hydrolysis of ATP, therefore, is likely to be involved in the mechanism of tRNA binding to the A site of the 80 S ribosome. As far as wide substrate specificity and possible participation in tRNA interaction with the ribosome are concerned, the ribosomal ATPase seems to be similar to EF-3 found in fungi. A synergism in ATPase activities of yeast EF-3 and rabbit liver ribosomes at high ATP concentration and certain ribosomeEF-3 ratios have been observed. Rabbit liver ribosomes seem to stimulate the ATPase activity of yeast EF-3 similar to the mechanism in yeast ribosomes, though less efficiently.