Adenyl cyclase activity in intestinal membranes has been studied during development in the rabbit fetus from fetal day 17 to 10 days postnatally and in the human fetus from the 10th to the 17th wk of gestation. In the rabbit, the enzyme was already present by fetal day 17 and showed a fourfold peak rise in specific activity by 22 days. By 28 days, the specific activity had fallen toward adult levels and remained constant throughout gestation and the 1st wk of life. Fluoridestimulated activity showed a similar curve, and was 2.5-5 times the basal values. Activities in jejunum and ileum were comparable at all time points studied. Phosphodiesterase activity did not change during gestation. When fetal intestinal segments were incubated in vitro with purified cholera enterotoxin, adenyl cyclase activity in subsequently prepared membranes was increased two- to threefold. This level was not regularly further elevated by fluoride ion. Lithium ion inhibited both the basal and fluoride-stimulated enzyme activity in membranes prepared from rabbit fetuses at term. Lactase activity (reflecting the development of the microvilli) in either whole intestinal homogenates or in the membrane fractions showed a differnet pattern of development, with a rise beginning on fetal day 24 and a plateau just after birth. In intestinal membranes prepared from human fetuses, the activity of both basal and fluoride-stimulated adenyl cyclase tripled from the 10th to the 17th wk of gestation. The data both in the rabbit and in man show that intestinal adenyl cyclase is capable of responding to cholera enterotoxin quite early in gestation. In the rabbit, this occurs before the time of appearance or ville or of an enzyme marker (lactase) for microville. The results support the concept that adenyl cyclase is present in plasma membrane other than the brush border.