Highly significant (P smaller than 0.0025) increases in adenylate cyclase activity were seen at all fetal age periods (5-17 weeks) whenever sodium fluoride (5-10 mM) was added to the enzyme prepared from human myocardium. Norepinephrine (NE) at 10-4 M significantly elevated adenylate cyclase activity commencing at 6-7 weeks (P smaller than 0.01). Beginning at 8-9 fetal weeks, glucagon (6x10-6 M) effectively activated adenylate cyclase. Other hormonal agents, namely, histamine, epinephrine, and isoproterenol at 10-4 M, demonstrated an ability to activate the enzyme (P smaller than 0.025) by as early as 6-7 weeks and continued to act in this manner throughout the remainder of the developmental periods investigated. The beta blocking agents, propranolol, significantly inhibited (P smaller than 0.25) the stimulation of adenylate cyclase by NE throughout the 8-15 fetal week periods.