SUMMARY The concentrations of prostaglandin E2 and F2α have been measured by radioimmunoassay in portions of cord, placenta, amnion, chorion, decidua and myometrium. The samples were obtained at defined periods of pregnancy, and the results have been compared with those obtained from the analyses of endometrial and myometrial tissue removed from women during the secretory phase of a menstrual cycle. The results showed that during pregnancy the mean concentration of prostaglandin E2 was higher (27–518%) than the corresponding value for prostaglandin F2α in all tissues. At term the concentration of prostaglandin E2 (ng/100 mg wet weight of tissue, mean ± S.D.) was higher in the umbilical cord 5·54 ± 0·88), decidua (4·02 ± 1·78) and myometrium (4·19 ± 1·06), than in the amnion (2·25 ± 1·27), chorion (1·64 ± 0·63) or placenta (1·04 ± 0·25). During labour there was a significant rise (P < 0·0005, Student's `t' test) in the concentration in decidua (10·76 ± 4·45), and to a lesser extent (P < 0·05) in the myometrium (5·84 ± 2·65) and amnion (4·77 ± 2·51). The overall concentration in decidua during the first trimester (3·09± 1·02) was significantly lower (P < 0·005) than in endometrial tissue (16·82 ± 10·13). The concentration was lower in myometrial tissue from non-pregnant subjects (2·90 ± 2·21), than in the corresponding tissue removed at term (4·19 ± 1·06) or during labour (5·84 ± 2·65). The results for prostaglandin F2α showed a similar pattern, but the values were significantly lower in the umbilical cord, and the percentage changes in concentration in the decidua and myometrium were of a higher magnitude.