The effects of repetitive administration of 50 micrograms of LRH and 100 micrograms of TRH in a single bolus at 0, 90 and 180 min on Prl, TSH, FSH, beta-hCG and oestriol were tested in 11 healthy pregnant women in their third trimester of pregnancy. Basal hormonal levels were in agreement with previous reports. The Prl and TSH peaks after each injection of LRH/TRH were similar for each hormone. No significant changes in serum FSH, beta-hCG and oestriol were detected after repetitive administration of LRH/TRH. Thus, the responsiveness of lactotroph and thyrotroph cells to pulsatile stimulation with LRH/TRH is not altered during the last trimester of pregnancy, and FSH, beta-hCG and oestriol levels remain constant during the period of sampling.