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 estrus 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.