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The physiological indexes of the copulatory behavior of the stallion have not been investigated in detail and may differ from those of other species, such as humans and rats.In order to understand the breeding capability of various stallions, their behavior during copulation was observed, and heart rate (HR) and the plasma concentrations of norepinephrine (NA) and epinephrine (Ad) were measured sequentially for a total of 13 copulations carried out during 2 days.The mean HR at rest was 35.3 ±0.9 beats per minute (b.p.m.) and it peaked during mounting (162.1 ±5.4 b.p.m.). The HR at ejaculation was 145.7 ±5.1 b.p.m, which was less than the peak. The plasma concentrations of NA and Ad showed similar changes to the HR; immediately after ejaculation they were, respectively, 4.7-fold and 1.9-fold higher than the resting values and there was a difference in the degree of increase of each catecholamine.The present results show that in the stallion the HR peaks at mounting and there is a greater change in the activity of the sympathetic nervous system because of the shortterm, highly intense exercise performed during copulation. It is considered that this, combined with the particular mental stress placed on the stallion during copulation, has the potential to cause sudden cardiac death.