In male dogs, there is no seasonal period of increased or decreased sexual activity. Rather, they can be stimulated at any time by nearby females that are in season (heat).
In the male, the important structures of the reproductive system are the testicles, ductus or vas deferens, prostate gland, and penis. Sperm production and storage occurs within the testicles. Upon ejaculation, the sperm is transported to the prostate gland by the vas deferens. Within the prostate, additional fluids are added to the sperm to nourish it and aid in its transport from
the penis and through the uterus.
The sperm and prostatic fluids, at the level of the prostate gland, enter the common urethra and are carried from the body through the penis. The penis of the dog has two specialized structures. The glans penis is a bulb-like dilation at the base of the penis, which fills with blood and holds the penis within the vagina during intercourse. Within the penis is a bone that maintains the shape and direction of this organ during mating. The penis is protected from the environment, as it is enclosed within the sheath or prepuce.
The more common disorders of the reproductive tracts of puppies include vaginitis in females, and cryptorchidism and phimosis in males