With the advent of assisted reproductive technology, pregnancy has become possible without sperm. The nucleus of the round spermatid, which contains a complete haploid set of chromosomes, can be used as a substitute for spermatozoa in animals and humans. In the mouse, round spermatids from surgically induced cryptorchid and prepubertal testes can fertilize normally and develop into normal offspring. The spermatid nucleus develops into a large pronucleus only when the oocyte is activated by artificial means, such as electrostimulation and oscillogen injection. Nuclei of mouse primary spermatocyte have been injected into oocytes and found to undergo meiosis, form an embryo and produce live young. Intracytoplasmic injection of male immature germ cells may become available as a treatment for patients with azoospermia due to maturation arrest.