Circadian pattern of prolactin and growth hormone levels in female rats of two strains.

Plasma growth hormone and prolactin levels were determined by radioimmunoassay at intervals of 2 hr during the light phase of a controlled light-dark cycle in virgin female (COBS F/Crl BR) and [Sch:SPD(SD)BR] rats. Peak prolactin concentrations were observed in the afternoon during proestrus and estrus in both rat stocks. Growth hormone exhibited wide fluctuations in the Sch:SPD rat, attaining peak levels at 1300 hr, but no apparent periodicity was observed in the COBS rats.