The clinical and pathological features of 21 ovarian germ cell tumours (14 dysgerminomas and 7 teratomas) from four mammalian species are presented. All the dysgerminomas were composed of a monophasic population of cells with a variable type of stromal response, depending upon the species, and 11 of the 14 were confined to the ovary. Only one dysgerminoma was associated with teratomatous elements. The teratomas were relatively solid, less cystic than those of women, and mature tissue predominated, although solid teratomas contained immature elements capable of metastasis. There were no examples of embryonal carcinoma or choriocarcinoma such as occur in women. From our observations and those of others, coupled with experimental data, it is concluded that germ cell tumours may be divided on the basis of their potential for differentiation. Those failing to express any potential for differentiation persist essentially unchanged as either dysgerminoma or seminoma. Other germ cell tumours have some potential for tissue differentiation. That potential may lead to a very restricted and immature differentiation, such as embryonal carcinoma or choriocarcinoma as in women. Beyond the primordial germ cell stage, germ cell tumours in other mammals primarily manifest somatic differentiation (teratomas). Among the tumours exhibiting differentiation, the clinical behaviour tends to parallel the degree of differentiation. At one end of the germ cell spectrum is the primitive and highly malignant embryonal carcinoma, and at the other is the benign adult cystic teratoma.