The ability of decidual tissue (DT) to maintain progesterone secretion was tested by oberving the effect of ergocornine (ECO) on the serum progesterone level 24 hr after treatment, and on the duration of the post-treatment diestrus in pseudopregnant rats bearing decidualized uteri. ECO, which was used to suppress prolactin release, was given as a single subcutaneous injection of 1.0mg of the free base in 0.25 ml of 70% ethanol, on either day 6, 7, 8, or 9 of pseudopregnancy (PSP)(day 1 = day of ovulation). Controls, which were otherwise treated in exactly the same way, included:normal (ordinary) PSP rats; rats hysterectomizedon either day 5 of PSP or several cycles before induction of PSP; PSP rats whose uteri were slit lengthwise on day 8 or 9; and DT-bearing rats in which the DT was removed immediately before the ECO injection. There was-no significant differenceamong any of the control rats in either criterion ofprogesterone secretion; their mean (‡SEM) post treatmentdiestrus was 2.2 ‡ 0.1 days (N = 81) and their mean serum progesterone level, 21.7 ‡2.1ng/ml (N = 47). Among the DT-bearing rats, the post-treatment diestrus was 6.8 ‡ 0.5 days (N =76), and their progesterone level was 44.5 ‡3.8 ng/ml (N = 51). Day of ECO treatment did not influencethe controls2 response to ECO, and affected the DT-bearing rats only slightly. The progesterone levels represented about 60% and 30% of the expected values for untreated DT-bearing and controlrats, respectively. In 11 DT-bearing rats the progesterone level 72 hr after ECO treatment(50.8 ‡ 9.2 ng/ml) was not significantly differentfrom the 24-hr value. The post-treatment diestrus accuratelyreflected the interval to ovulation. There wasno evidence by radioimmunoassay for prolactin-likematerial in an extract of DT. The findings suggest that DT may produce a hormone which can sustain progesterone secretion in the relative absence of prolactin.(Endocrinology95: 1113, 1974)