By regarding the vacuum as a perfect fluid with equation of state p =
-rho, de Sitter's cosmological model is quantized. Our treatment diffe
rs from previous ones in that it endows the vacuum with dynamical degr
ees of freedom, following modern ideas that the cosmological term is a
manifestation of the vacuum energy. Instead of being postulated from
the start, the cosmological constant arises from the degrees of freedo
m of the vacuum regarded as a dynamical entity, and a time variable ca
n be naturally introduced. Taking the scale factor as the sole degree
of freedom of the gravitational field, stationary and wave-packet solu
tions to the Wheeler-DeWitt equation are found, whose properties are s
tudied. It is found that states of the Universe with a definite value
of the cosmological constant do not exist. For the wave packets invest
igated, quantum effects are noticeable only for small values of the sc
ale factor, a classical regime being attained at asymptotically large
times.