Abstract
Peripheral plasma levels of progesterone, cortisol, and corticosterone were measured every other day throughout pregnancy in primiparous rabbits by competitive protein-binding techniques. Progesterone concentrations rose immediately after breeding and reached peak levels on Days 16–18, after which there was a decrease to prepartum levels by Day 32. Corticosterone remained constant for the first 24 days of gestation (7.3 ng/ml) with a slight rise to 17.0 ng/ml during the last few days of pregnancy. Cortisol levels (7.1 ng/ml) also remained constant for the first 24 days of gestation, although, in contrast to corticosterone, an abrupt increase began on Day 26 with peak levels (96.7 ng/ml) on Day 30. By Day 32, cortisol levels were similar to those in early pregnancy. Declining progesterone levels plus increased cortisol levels appear to be important preparturient changes in the rabbit.
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