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.

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