  We report the discovery of spectroscopic variations in GD 323, the
prototypical DAB white dwarf. Simultaneous optical spectroscopic observations
over five consecutive nights of GD 323 and of PG 1234+482, a non-variable
comparison DA white dwarf of similar brightness, are used to reveal
quasi-periodic variations in both the hydrogen and helium absorption lines over
a timescale of hours. The amplitude of the variation of the equivalent width of
Hbeta is ~30 %. Moreover, the strength of the hydrogen lines is shown to vary
in opposite phase from that of He I 4471. These results suggest that the model
currently thought to be the most viable to account for the simultaneous
presence of hydrogen and helium lines in GD 323, namely a static stratified
atmosphere, may need to be reexamined. Instead, a model with an inhomogeneous
surface composition, resulting perhaps from the dilution of a thin hydrogen
atmosphere with the underlying helium convection zone, may be a better
representation of GD 323. The observed variation timescale of ~3.5 hours is
consistent with the slow rotation rate of white dwarf stars.
