The present invention provides novel compositions and methods for using the S(+) enantiomer of desmethylselegiline (N-methyl-N-(prop-2-ynyl)-2-aminophenylpropane), for the treatment of selegiline-responsive diseases and conditions. Diseases and conditions responsive to selegiline include those produced by neuronal degeneration or neuronal trauma and those due to immune system dysfunction. Effective dosages are a daily dose of at least about 0.015 mg/kg of body weight.