The foetal testis of the field rat (Millardia meltada) shows seminiferous tubules and interstitium consisting of mesenchymal cells and differentiated Leydig cells associated with blood vessels. The tubules during the prenatal period show gonocytes and Sertoli cells. After birth, their diameter decreases but again increases progressively after postnatal days 9 and 10. Spermatogonia appear among the gonocytes on postnatal day 1 and primary spermatocytes on day 8. Secondary spermatocytes and spermatids are not seen up-to-day 23. Several hypertrophied Leydig cells are seen in the foetal testis on day 17 but are greatly increased in number on days 18 and 19. A few hours after birth the Leydig cells show a rapid decrease in their number. These fluctuations in the Leydig cells of prenatal and neonatal testes have been correlated to the rise and fall in testosterone production during these periods. The Leydig cells show the histochemical characteristics of actively steroid-secreting cells, which consist of the presence of diffuse lipoproteins and a few lipid granules consisting of phospholipids; no cholesterol and/or its esters could be demonstrated. Such lipids are not present in the cytoplasm of undifferentiated mesenchymal cells. The seminiferous tubules do not show any appreciable development of lipid changes.