In order to compare the nocturnal penile erection and the erection induced by intracavernosal injection of papaverine, digital palpation of penis, erectile angle, and the rigigram using $Rigiscan^{(R)}$ were evaluated in 38 erectile dysfunction patients(13 psychogenic, 18 vasculogenic, 1 neurogenic, 3 vasculo-neurcgenic, and 3 undetermined). The following results were obtained. 1. Among the 32 patients of the nocturnal penile erection > 40% on rigigram, only 15 patients(46.9 %) demonstrated rigidity > 4+, a sufficient rigidity for sexual intercourse, on papaverine test. 2. The maximal degree of rigidity was also measured by the erectile angle of the penis. Among the 32 patients of nocturnal penile erection > 40% on rigigram, 23 patients(71.9%) had an angle > $45^{circ}$. 3. Sixteen patients(42.1%) demonstrated similar increase in the maximal circumferential expansion of the distal portion of the penis on nocturnal penile erection and papaverine-induced erection. Nineteen patients showed larger circumferential expansion on nocturnal penile erection than on papaverine-induced erection. Fifteen patients showed similar circumferential increase of the basal portion of the penis on nocturnal penile erection and artificial erection, while 12 patients(31.6 %) showed larger circumferential increase on artificial erection than on nocturnal penile erection. 4. In the 15 patients of nocturnal penile erection > 40 % on rigigram and > 4+ on papaverine test, the duration of maintenance of the maximum erection on nocturnal penile erection and artificial erection proved to be not proportional. Thus, the arthficial erection induced by papaverine and the nocturnal penile erection can not to be said to be of same in nature and therefore, in order to make more definite diagnosis, both tests should be done and compared.