Male house mice (Mus musculus) were killed between 10 and 100 days of age to establish the temporal relationships between the development of fighting behavior and fertility and the spermatogenic and steroidogenic functions of the testis. Testis growth was directly related to increases in the concentration of protein and carbohydrate and the ratio of RNA/DNA in testicular homogenates, with the most obvious changes being noted between 21 and 55 days of age. Plasma androgen concentrations increased 300% during that time period and then declined 50% by 100 days of age. Importantly, increases in plasma androgen concentration paralleled increments in the numbers of Leydig cells and step 7 spermatids. Spermatogenesis was completed qualitatively by 35 days of age, and spermatozoa were detected at the cauda epididymis—vas deferens juncture at 40 days of age. Isolation of [14C] androgens following in vitro incubation of teased testis tissue with [1-14C]acetate demonstrated that 80% of the radioactive products were associated with [14C]androstenedione and 5-α-reduced androgens between 10 and 40 days of age, while 80% was associated with [14C]testosterone after 55 days of age. Intermale aggression was first observed at 35 days in 90% of the individuals tested. In comparison, viable mating was first apparent at 40 days and reached adult levels by 55 days of age. It was concluded that the development of viable mating in males encompasses a relatively broad time span that was closely tied to the maturation of the spermatogenic and steroidogenic functions of the testis, while aggressive behavior had a sharp onset that coincided with the first evidence of heightened androgen secretion.