Nuclear envelopes of mammalian cells contain a nucleoside triphosphatase which is probably involved in mRNA transport through the nuclear membrane. The activity of the enzyme, studied in RNA-depleted nuclear ghosts, can be stimulated by poly(A) or by poly(A) (+)mRNA. Using nuclear ghost preparations from mature (8-10 months' old) and old (40-42 months' old) Wistar rats, it was shown that in "old" preparations the basal activity of the enzyme is significantly reduced (by 15%). In addition, the enzyme from old animals responds only very little to poly(A) or poly(A) (+)mRNA, compared to preparations from mature animals. Using a concentration of 6.8 X 10(11) poly(A) (+) mRNA molecules per microgram of enzyme preparation, the nucleoside triphosphatase from mature animals is stimulated by 77% and the enzyme from old animals by only 26%. Binding studies of poly(A) to pore laminae revealed that the number of binding sites in unphosphorylated preparations from old animals is significantly reduced (by 24%) compared to "mature" preparations. As a consequence of in vitro phosphorylation, no difference is observable in the number of binding sites between the two age groups. The values for half-maximal saturation binding constants for poly(A) are identical in unphosphorylated and phosphorylated pore-laminae preparations, irrespective of the age group studied. The results presented indicate that in old animals the pathway from the phosphorylated to the dephosphorylated nuclear-envelope protein which is controlled by poly(A) is impaired in the proposed cycle for mRNA efflux from nuclei.