Summary. Young male rats were exposed to Busulphan on selected days between the 5th day of foetal life and Day 15 post partum, inclusive. The drug apparently had no effect on the foetal sex cells when administered to the pregnant female between Days 5 and 7 post coitum. Later in pregnancy a destructive action became rapidly more powerful. The developing sex cells of the foetus were destroyed after exposure to the drug from Day 13 post coitum onwards. Supporting cells, which proliferate simultaneously with the developing sex cells, appeared to be quite indifferent to this treatment with Busulphan. Treatment during the neonatal period resulted in a selective effect on the germ cells. It was found that the sensitivity of these cells decreased rapidly after birth. The relationship between testicular damage and fertility is discussed.