Endogenous concentrations of prostaglandin (PG) F2 alpha and PGE2, but not of 6-oxo-PGF2 alpha, in the uterus were significantly higher in ovariectomised, control rabbits than in ovariectomised rabbits which received oestradiol and/or progesterone treatment. However, due to significant increases in uterine weight following steroid treatment, there were no significant differences in the total uterine content of PGF2 alpha or PGE2 following hormonal treatment, whereas total 6-oxo-PGF1 alpha content in the uterus was significantly increased after oestradiol treatment when given alone or following progesterone priming. When PG synthesis by uterine homogenates in vitro was examined, expressing the results on a unit weight basis showed that oestradiol treatment alone significantly depressed the production of PGF2 alpha and PGE2, but not of 6-oxo-PGF1 alpha, when compared to ovariectomised, control rabbits. Progesterone treatment had no significant effect, and reversed the effect of oestradiol on PGF2 alpha synthesis. When the results were expressed on a total uterine basis, however, oestradiol, acting on a progesterone primed uterus, was the optimum stimulus for maximum PGF2 alpha synthesis. Total PGE2 production was not significantly affected by steroidal treatment, whereas oestradiol, alone or following progesterone priming, significantly increased 6-oxo-PGF1 alpha production. These findings are discussed in relation to the stimulus necessary for increased PGF2 alpha secretion from the rabbit uterus at the end of pseudopregnancy.