The release of lactogenic hormone (LGH) was studied at the onset of pseudopregnancy in 90 rats. The LGH-content of the pituitary glands was determined by the pigeon crop sac assay. Glass rod stimulation of the cervix during vaginal oestrus was followed by a significant decrease in the LGH-content of the pituitary glands, thus demonstrating a correlation between LGH-release and beginning pseudopregnancy. Anaesthesia by Nembutal almost prevented this effect of cervical stimulation.

Temporal aspects of the decidual cell reaction (DCR) were studied in 331 pseudopregnant (lactogenic hormone treated) rats under rhythmical and permanent illumination. The peak of uterine sensitivity to the trauma in rats, exposed to light for 14 hours/day was found at 2 p. m. on day 4, and in rats, exposed to permanent illumination at 6 a. m. on day 4 of pseudopregnancy. Rats, traumatized during the decreasing phase of sensitivity, only showed the DCR restricted adjacent to the cervix. Standardized light exposure is postulated for studies on the DCR.

The decidual cell reaction (DCR) was studied in a total of 60 rats. Pseudopregnancy was induced during vaginal (late) oestrus, either by cervical glass rod stimulation or by daily injections of 1 mg of a lactogenic hormone (LGH) preparation. The antimesometrial aspect of the right uterine horn was traumatized on day 4 of pseudopregnancy. All animals of both groups reacted with massive and almost identical deciduoma formation, as measured by dry fat free tissue (DFFT) and histological examination.

For LTH-determination in biological material without preliminary extraction procedures the pigeon crop sac assay, performed by the micro-method of Grosvenor & Turner (1958), and modified by the addition of prednisolone to the standard or test material was used. Serum samples of lactating women caused no inflammations of the skin of the pigeons, and the crop sacs hardly showed any signs of inflammation. Reaction sites could be measured more easily, and the LTH activities in the plasma were about 3 times higher than in plasma processed by the acetone extraction method of Sulman (1956).