1. 216 mice, spayed on the 14th day of their first pregnancies, were injected daily thereafter with progesterone, either alone, with oestradiol or relaxin extracts or both; no injections were given after parturition. Extracts of relaxin obtained from pregnant rabbit serum and pregnant sow ovaries were used.
2. When given alone, 1 mg/day progesterone maintained pregnancy to full term in 83%, and 0·5 mg/day in only 30% of mice. Parturition was often delayed, prolonged and difficult, with consequent death of the foetus in utero, even when injections were stopped on the 18th day. A high proportion of young were stillborn, and none were reared. The symphysis pubis remained closed and cartilaginous.
3. 1·5 μg/day oestradiol did not synergize the action of progesterone in maintaining pregnancy, but there were fewer delayed deliveries and a higher incidence of live births; a small number of mice reared some of their young. The innominate bones did not separate, but the symphysis was less rigid than when progesterone was given alone.
4. Relaxin extracts had a synergizing action when all three hormones were given together, 0·5 mg/day (but not 0·25 mg) of progesterone then being enough for maintenance of pregnancy in over 80% of mice. The experiments have not established whether oestradiol was necessary for this synergism. There was a positive relation between punctual and normal delivery of live young and potency of the relaxin extracts in producing full pregnancy changes in the pelvis. More mice reared some of their young. None of the hormone combinations used completely compensated for the absence of the ovaries.
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