Analysis of spectra of the quasar 3C 390.3 covering a period of over 20 yr shows that the blueshifted peak of H \beta has been changing its radial velocity at an almost constant rate during this time .
The radial velocity has increased by over 1500 km s ^ { -1 } .
The lower limit to the period of radial velocity changes is 210 yr .
Although very long periods can not be excluded by the radial velocity curve alone , other considerations suggest that the period is \sim 300 yr .
If the radial velocity changes are due to orbital motion , the radius of the orbit is \sim 0.3 pc and the total mass of the system is \sim 7 \times 10 ^ { 9 } M _ { \odot } .
In the binary black-hole model the masses of the two holes are \sim 2.2 \times 10 ^ { 9 } M _ { \odot } and \sim 4.4 \times 10 ^ { 9 } M _ { \odot } .
A possible third peak claimed to be present in some 1974-75 spectra is shown to be an instrumental artifact .
The narrowness of the displaced peaks in 3C 390.3 objects requires that the broad line region cloud motions are not governed solely by gravity .
The black hole masses derived by Koratkar & Gaskell ( 1991 ) need to be increased significantly .
This probably lowers accretion efficiences to less than 1 % of the Eddington limit .