Pluto's orbit has been integrated for 4.5 million years. The previously discovered libration of 3X-2XN- Co, where X and XN are the mean longitudes of Pluto and Neptune and Co is Pluto's longitude of perihelion, is confirmed and has an average period of 19 951 yr. It was also found that the argument of perihelion Co librates about 90 with an amplitude of 24 and a period of 3 955 000+20000 yr. There is an indication that both the difference between the nodes and the difference between the longitudes of perihelia of Neptune and Pluto may be locked on to the Co libration. All of the above effects are found to improve the stability of the system by increasing the minimum distance of approach between the two bodies.