Some experiments have been carried out in order to establish the effects on Triturus lampbrush chromosomes after treatment with H³ actinomycin D following two methods: 1) By incubating intact oocytes in the antibiotic, all the normal loops, which represent the overwhelming majority of the loops, gradually collapse into the chromomeres: these loops represent active sites in RNA synthesis. Some loops with dense, granular and fibrillar matrix (for instance the dense loops of T. alpestris apuanus chromosome I and the subterminal granular loops of chromosome XI) are not so sensitive to actinomycin D; in fact they undergo only a light retraction even after a very prolonged incubation: these loops are the same which in medium and large size oocytes show a lower level of incorporation of H³ uridine. Spheres, which perform no RNA synthesis but only protein synthesis, do not retract nor decrease in size even after several hours of incubation. The autoradiographic technique has revealed appreciable radioactivity on lampbrush chromosomes and nucleoli: this indicates that the retraction of the loops can be correlated with the binding of the antibiotic with the chromosomal DNA. 2) By incubating the nuclear content alone, no loop collapses and the morphology of lampbrush chromosomes appears to be unaffected. However radioactivity is clearly evident on chromosomes and nucleoli: therefore the retraction of the loops is probably also connected to the vitality and integrity of oocytes.