Macromolecular syntheses were studied by incorporation of 14C-labeled thymidine, 14C-uridine and 14C-amino acids, and by use of specific inhibitors of RNA, DNA, and protein synthesis upon the rate of regeneration of post-peristomial pieces of Blepharisma intermedium. The effect of these inhibitors on the macronuclear cycle which occurs during regeneration was also determined. Mitomycin, naladixic acid, and pheleomycin—inhibitors of DNA synthesis—if in relatively high concentration and applied for a long time, delayed regeneration. 14C-Thymidine was incorporated into most fragments of unsynchronized cells slowly during regeneration, although it was incorporated actively into blepharismas just before division. Naladixic acid and phleomycin reduced 14C-thymidine incorporation during regeneration. 14C-Thymidine was not incorporated into macronuclei of cells cut soon after division (synchronized cells). 14C-Uridine was incorporated actively immediately after removal of the peristome and continued throughout regeneration. The label at first was strongest in or over the macronucleus and near it; it was weak in the cytoplasm, becoming stronger with lapse of time after cutting. Actinomycin D, which retards regeneration most actively immediately after cutting, reduced incorporation of 14C-uridine. 14C-Histidine, 14C-leucine and 14C-amino acids (mixture) were incorporated into regenerating blepharismas. Puromycin retarded regeneration of Blepharisma most actively 1 to 2 h after cutting. Puromycin of the concentration which retards regeneration reduced the incorporation of 14C-amino acids. The results suggest that cutting Blepharisma incites active RNA synthesis (mRNA?) in the macronucleus. The spread of RNA in the cytoplasm is accompanied by protein synthesis. The experiments suggest that macronuclear DNA synthesis is not essential to regeneration, but probably occurs when the cells are cut at the time they are in or near the synthetic stage.