The effect of RNA synthesis inhibition on the progress of mitosis was studied in meristematic cells ofAllium cepaat the ultrastructural level. The continuous treatment with 3'AdR and EB on cells which are in the beginning of mitosis, causes the persistence of the nuclear envelope (GONZALEZ-FERNANDEZet al.1970) without blocking the chromosomal cycle (RISUEÑOet al.1975). In the case of EB there are ruptures in some points of the nuclear envelope but with 3'AdR it is still intact.