Instillation of oil into progesterone-primed, oestrogen-sensitized uteri of mice resulted in secretion patterns which were similar, but not identical, to those found on Day 5 of pregnancy. Stimulus-dependent responses common to pregnancy and the experimental decidual cell reaction included an early but transient increase in a 40 000 Mr basic protein and decreases in two other proteins. Some of the characteristic changes were also found after oil instillation in the progesterone-maintained 'non-receptive' uterus, even though subsequent decidualization did not occur. Instillation of cholera toxin, another deciduogenic substance, also resulted in patterns similar to those of pregnancy, including an increased secretion of a 25 000 Mr acidic protein which was only minimally produced during the oil-induced decidual cell reaction.