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ExtractRenal cortex from fetal and neonatal rabbits (from 4-5 days preterm to 4 days of age) shows results comparable with maternal renal cortex during in vitro stimulation of the 2,200 X g pellet with parathyroid hormone (PTH) at 20 /ug/ml. Incubation with sodium fluoride (10 HIM), a nonspecific potent stimulator of adenyl cyclase, resulted in comparable increases in both the fetal and the maternal renal cortex. Glucagon, prostaglandin Ei, and vasopressin, all at 20 jug/ml, stimulate adenyl cyclase to a lesser extent than PTH. Homogenates of full term neonatal rabbits and maternal renal cortex show comparable phosphodiesterase enzyme levels with or without 0.04 M imidazole. These data show that in fetal and early neonatal renal cortex there is no developmental adenyl cyclase responsiveness to PTH. Similarly, phosphodiesterase is present equally in term and maternal rabbit renal cortex. Speculation

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