Mediated transport across the nuclear envelope was investigated in proliferating and growth-arrested (confluent or serum starved) BALB/c 3T3 cells by analyzing the nuclear uptake of nucleoplasmin-coated colloidal gold after injection into the cytoplasm. Compared with proliferating cells the nuclear uptake of large gold particles (110-270 A in diameter, including the protein coat) decreased 5.5-, 33-, and 78-fold, respectively, in 10-, 14-17-, and 21-d-old confluent cultures; however, the relative uptake of small particles (total diameter 50-80 A) did not decrease with increasing age of the cells. This finding suggests that essentially all pores remain functional in confluent populations, but that most pores lose their capacity to transport large particles. By injecting intermediate-sized gold particles, the functional diameters of the transport channels in the downgraded pores were estimated to be approximately to 130 and 110 A, in 14-17- and 21-d-old cultures, respectively. In proliferating cells, the transport channels have a functional diameter of approximately 230 A. The mean diameters of the pores (membrane-to-membrane distance) in proliferating and confluent cells (728 and 712 A, respectively) were significantly different at the 10%, but not the 5%, level. No differences in pore density (pore per unit length of membrane) were detected. Serum-deprived cells (7-8 d in 1% serum or 4 d in 0.5% serum) also showed a significant decrease in the nuclear uptake of large, but not small, gold particles. Thus, the permeability effects are not simply a function of high cell density but appear to be growth related. The possible functional significance of these findings is discussed.