Grizzly bears (Ursus arctos horribilis) in Montana and Wyoming killed between 1978–95 reached puberty between the ages of 3.5–5.5 years, consistent with similar studies on brown bears in Hokkaido (Japan), Alaska, and Yukon. Our study considers age-related variation in testicular and epididymal characteristics for 14 pre-pubertal and 11 post-pubertal grizzly bears. Testis mass increased 200% in bears between 1.5 and 7.5 years old and reached an asymptote at 10.5 years. Similarly, seminiferous tubule mass increased rapidly in bears between 1.5 and 5.5 years of age and reached an asymptote in 10.5 years. Testis mass and volume, seminiferous tubule and interstitial volume, and seminiferous tubule diameter were smaller in pre-pubertal bears than in post-pubertal bears. Seminiferous tubule volume increased rapidly between 1.5 and 7.5 years of age. Seminiferous tubule volume reached an asymptote at approximately 10.5 years of age. Interstitial tissue volume increased rapidly between 1.5 and 3.5 years of age and reached an asymptote at approximately 6.5 years of age. Epididymal mass was smaller in pre-pubertal than in post-pubertal bears. Epididymal tubule diameters, percent areas occupied by the tubule, the lumen of the tubule, and the muscularis and epithelium of the tubule did not differ between pre- and post-pubertal bears. The epididymis:testis mass ratio did not differ between pre- and post-pubertal bears. Epididymal mass was linearly related to testis mass. Epididymal tubule diameter increased from the caput, through the corpus, to the cauda. Percent tubular area of the caput was smaller than either the corpus or cauda. Percent tubular areas occupied by the lumen and the muscularis and epithelium did not differ among the caput, corpus, and cauda. This was the first detailed study of the effects of age on testicular and epididymal characteristics of grizzly bears in the continental US.