Marsupials differ from most other mammals in their method of reproduction, in that they have chosen, in an evolutionary sense, to develop lactation rather than placentation for the nurture of their young. The neonate is therefore born with a mixture of advanced and embryonic characters, and yet is readily accessible within the pouch, providing a unique system for the study of the ontogeny of various physiological and endocrinological parameters. Marsupials are therefore ideal animals for research into mammalian reproductive physiology. The results of this exciting new research are summarized...