Cyclic[14C]adenosine 3′,5′-monophosphate (cyclic AMP) in human lung fragments has been measured by an isotopic method. It was found that aminophylline transiently increased cyclic[14C]AMP by 100% in doses above 1 mM. β-Adrenergic agents, epinephrine and isoproterenol, increased cyclic [14C]AMP by greater than 1000% through at least 10 min of incubation. Combinations of isoproterenol and aminophylline acted synergistically. Norepinephrine alone increased cyclic[14C]AMP but to a lesser degree than isoproterenol or epinephrine. In the presence of β-adrenergic blockade, norepinephrine and epinephrine decreased cyclic [14C]AMP below control values. © 1971.