A simple formula is derived to calculate the nucleocytoplasmic RNA-Efflux per nuclear Pore complex per min (REP-rate) which is generally applicable both for "growing" and "stationary" eukaryotic cells. In actively growing cells this REP-rate is mainly dependent on the cytoplasmic RNA-pool, the number of RNA-transporting pores, and the growth constant of RNA. These parameters are determined in logarithmicTetrahymena pyriformis GL. In this organism, 45 molecules both of the larger ribosomal RNA (25s rRNA) and of the smaller (17s rRNA) are transported per pore per min from nucleus to cytoplasm. "Pulse-label" experiments with(3)H-uridine indicate that the 25s rRNA is obviously transferred more slowly to the cytoplasm than the 17s rRNA. We postulate a "gating hypothesis" on the regulation of the nucleocytoplasmic RNA-efflux by nuclear pore complexes. This gating hypothesis suggests that nucleopores are controlling points of secondary importance in the sequence of gene expression, and do not directly control the cytoplasmic protein synthesis in eukaryotic cells.