  We report a detailed study of the magnetic-field-orientation dependence of
the millimetre-wave magnetoconductivity of the superconductor Sr2RuO4 We find
two harmonic series of cyclotron resonances. We assign the first, corresponding
to a quasiparticle mass of $4.29 \pm 0.05 m_{\rm e}$, where $m_{\rm e}$ is the
free-electron mass, to the $\beta$ Fermi-surface section. We assign the second
series, which contains only odd harmonics, to cyclotron resonance of the
$\gamma$ Fermi-surface section, yielding a quasiparticle mass of $12.35 \pm
0.20 m_{\rm e}$. A third, single cyclotron resonance, corresponding to a
quasiparticle mass of $5.60 \pm 0.03 m_{\rm e}$, is attributed to the $\alpha$
Fermi-surface section. In addition, we find a very strong absorption mode in
the presence of a magnetic field component parallel to the
quasi-two-dimensional planes of the sample. Its dependence on the orientation
of the magnetic field cannot be described in the context of conventional
cyclotron resonance, and the origin of this mode is not yet clear.
