  High-field electron spin resonance (ESR) experiments have been carried out in
single crystals of La_{1-x}Sr_xMnO_3 in the concentration range 0 \le x \le
0.175. Different quasioptical arrangements have been utilized in the frequency
range 40 < \nu < 700GHz and for magnetic fields B \le 12T. A splitting of the
antiferromagnetic resonance (AFMR) mode is observed in the magnetic field for
the parent compound LaMnO_3 in agreement with the antiferromagnetic structure
of this material. Abrupt changes in the AFMR frequencies have been observed
around x = 0.025 and attributed to the possible transition between
antiferromagnetic and canted state. For increasing Sr-doping the observed AFMR
modes are splitted even in zero field, which can be naturally explained using a
concept of a canted magnetic structure for x<0.1. In La_{0.825}Sr_{0.175}MnO_3
the ESR spectra are consistent with the ferromagnetic and metallic state. The
lines of ferromagnetic resonance and ferromagnetic antiresonance can be clearly
seen in the spectra. For intermediate concentrations 0.1 \le x \le 0.15 a
complicated set of ESR spectra has been observed, which can be well explained
by a single ferromagnetic resonance mode and taking into account electrodynamic
effects.
