It was determined that change in DNA content in macronuclei occurs in the T. pyriformis infusoria under the influence of an activator (caffeine) and inhibitors of Ca2+ channels (verapamil), NiCl2, and CdCl2. Caffeine (10 mM) stimulates DNA synthesis. Verapamil (5 microM), CdCl2 (125 microM), and NiCl2 (100 microM) decrease DNA content in macronuclei by 30 min after proliferative stimulation. By 4 h of incubation, there is, on average, 10% less DNA in macronuclei of Tetrahymena preprocessed with verapamil than in the control cells. The cells preprocessed with CdCl2 and NiCl2 differ from the control cells by lower DNA content almost at all studied periods, but they restore the level of nuclear DNA by 4 h. It is assumed that transmission of proliferative signals in the T. pyriformis has a Ca2+ -dependent character.