Influence of season on mean plasma levels of prolactin, placental lactogen hormone and luteinizing hormone during the second half of gestation in the ewe.

Mean plasma levels of prolactin (PRL), placental lactogen (OCS) and luteinizing hormone (LH) have been studied in pregnant ewes at different times of the year from approximately day 60 of gestation until parturition. Three groups were constituted and inseminated at different times: September, group I; November, group II; March, group III. Blood samples were collected on two consecutive days approximately every 10 days. The results indicate that plasma PRL levels are influenced by the season. In group I, PRL concentrations were 3 and 2 times less than those of group III during the third and fifth months of gestation. OCS levels, strongly dependent on the stage of gestation, were not significantly affected by the season. LH concentrations remained low in all three groups throughout the studied period of gestation.