Heat shock, deciliation and release from anoxia result in similar alterations in the pattern of protein synthesis in starved Tetrahymena pyriformis. In each case, synthesis of the same set of (at least) 16 polypeptides is induced, and many of these polypeptides accumulate in stainable amounts within 50 min. The molecular weights of these proteins, which we refer to as stress proteins (sp), are very similar to those of the heat shock polypeptides of Drosophila. Heat shock and deciliation also lead to similar changes in proteins associated with isolated nuclei. One stress-induced polypeptide, designated sp29c, is highly enriched in the nucleus. This protein is undetectable in control cells but is synthesized in response to stress and accumulates in the nucleus in stainable amounts within 50 min. It is not released by staphylococcal nuclease digestion, suggesting that it is not chromatin-associated. Three other stress-induced proteins, sp73 and sp75a and b, are also present in nuclei isolated from stressed cells but, unlike sp29c, are not enriched in this compartment. Another protein, which is present in stainable quantities in the cytoplasm of control cells, appears to be translocated to the nucleus after stress.