SUMMARY

The oestradiol-induced increase of ornithine decarboxylase (ODC) activity in the 'withdrawn ' chick oviduct was found to be inhibited by progesterone. In vitro (2 h at 37 \s=deg\C), progesterone (1 \g=m\mol/l)abolished the effect of oestradiol (20 nmol/l), progesterone alone having no effect. In vivo, progesterone (3 mg/kg) inhibited \m=~\70%of the augmentation of ODC activity induced in the oviduct within 2 to 6 h of treatment with oestradiol benzoate(1\m=.\5mg/kg). Administration of progesterone alone in vivo caused an increase in the ODC activity, the maximum level measured after 6 h being similar to that obtained when the chicks were given both oestrogen and progesterone. In the rat uterus in vivo progesterone also inhibited the rise of ODC activity caused by oestradiol, \m=~\70%inhibition being observed between 2 and 6 h after treatment. Progesterone alone had no effect on uterine ODC activity during this period