The effect of yohimbine, an alpha-2 adrenoceptor antagonist, on diminished ejaculatory capacity induced by frequent ejaculation was investigated in dogs. Ejaculation was elicited by manual penile stimulation (for 5 min) 5-8 times every 30 min. The amount of ejaculate produced by the stimulation was drastically reduced during a period of frequent ejaculation in a frequency-dependent manner. When administered immediately after the first ejaculation, yohimbine (0.1 mg kg-1, i.p.) completely prevented the decrease in the amount of ejaculate produced in the succeeding ejaculation. Moreover, similar treatment with yohimbine immediately after the fifth ejaculation when the dogs displayed a greatly reduced ejaculatory capacity restored the capacity to the initial level. In addition, yohimbine increased the total number of sperm produced during a period of frequent ejaculation. These data corroborate our previous finding that yohimbine at a low dose has a stimulatory effect on ejaculatory function in dogs. The present study also indicates that an alpha-2 adrenergic mechanism may be involved in the diminution of ejaculatory capacity induced by frequent ejaculation.