Clinical reports have suggested that antidepressant medication may contribute to the sexual dysfunction experienced by some depressed patients. A double-blind trial in a non-psychiatric male population compared amitriptyline (tricyclic), mianserin (tetracyclic) and placebo for their effects on nocturnal sexual arousal. In a three-way crossover design active drug or placebo were taken for two weeks preceding measurement of the frequency, amplitude and duration of nocturnal penile tumescence and synchronous sleep indices. Both active compounds significantly decreased the amplitude and the total duration of nocturnal erections. The effects on sleep indices were as previously reported. Few differences were found between the tricyclic and tetracyclic drugs. Some implications of these findings are considered.