Semen output characteristics of 7,858 ejaculates of 57 commercial Holstein sires in artificial insemination were analyzed to estimate effects of environment over 3-yr. Large differences existed between bulls, first and second ejaculates, and intervals between collections for all measures of semen output. Effects of age of bulls were greatest for sperm concentration per milliliter and total sperm output, peaking at 4 yr of age. Original semen volume was influenced less by age. December was the best month for total sperm production even though June and July were the months for most volume of seminal fluid production. Total sperm output was highest on Fridays and lowest on Wednesdays. Effects of temperature and humidity and changes in temperature and humidity on semen output were small and inconsistent. These results do not support the assumption that semen output could be increased in controlled environment conditions.