Some observations on the behaviour of hypothalamic enzymes during the time of blastocyst implantation in the rabbit.

1. Hypothalami from pregnant and non-pregnant rabbits were homogenized in sucrose and separated into fractions by differential and density-gradient centrifugation. 2. The fractions were examined for enzymes capable of inactivating oxytocin. 3. Enzyme activity was detected in the supernatant and one particulate fraction of both pregnant and non-pregnant animals. 4. The specific activity of the enzymes increased during early pregnancy, and coincided with the time of blastocyst spacing and implantation in the uterus.