The author discusses two areas of psychophysiological research that may be relevant to the diagnosis and treatment of sexual dysfunction: investigations of sexual arousal in men and studies of nocturnal penile tumescence. There has been no convincing evidence to date of a predictable, generalizable autonomic response pattern to sexual arousal, but neuroendocrine research in this area has shown promise. A close temporal relationship has been found between REM sleep and penile tumescence in normal men-deviations from this pattern may be related to sexual dysfunction. The author suggests that further research is necessary in these areas of potential diagnostic and prognostic significance.