Plasma progesterone concentrations in the pregnant guinea-pig are greater than in many species (Heap & Deanesly, 1966; Heap, Perry & Rowlands, 1967) and far higher than those found in the normal cycle (Table 1). The present experiments were intended to compare the effects of subcutaneous progesterone tablets on plasma progesterone levels after ovariectomy with those in normal, non-pregnant and pregnant females.
The ovaries are essential for pregnancy in most guinea-pigs between about Days 14 and 28 (Deanesly, 1963) but from about Day 28 pregnancy will continue normally without exogenous hormones owing to the production of placental progesterone. In this study it was decided to use females in the 1st month of pregnancy when the actual production of progesterone by the placenta is far less than that of the ovary and is well below that reached in later stages (Heap
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