Pineal glands in organ culture synthesize and release biopterin and are able to maintain concentrations of biopterin occurring in vivo for up to 54 hours in vitro. The intracellular biopterin content is reduced 50 percent by treatment with l-norepinephrine or cyclic adenosine monophosphate derivatives, but not by d-norepinephrine. This is an indication that biopterin levels are regulated by an adrenergic cyclic adenosine monophosphate-dependent mechanism. The decline in tissue biopterin content, produced mainly by inhibited of biosynthesis, is maximal at 6 hours and is not associated with either an increase in biopterin release or a shift in the reduction state of the biopterin.