The induction of pseudopregnancy in the rat abolishes, for about 8 days, the hypothalamic inhibition of pituitary luteotrophic hormone (lth) secretion (Zeilmaker, 1965). Pituitary lth is available throughout this period, whether functional corpora lutea are present or not. If a new oestrus is induced during this 8-day period, this oestrus is followed by a fresh pseudopregnancy of normal length, still benefiting, as it were, from the prior stimulus which initiated lth secretion and induced the first pseudopregnancy; the stimulus is `remembered' in the central nervous system (Everett, 1967).
Ergocornine interrupts progestation in the pregnant and pseudopregnant rat, and oestrus appears within 2 to 3 days of administration of the ergot alkaloid (Shelesnyak, 1955). This oestrus, caused by the interruption of gestation, is followed in 50 to 70% of cases by