Norepinephrine causes a four- to sixfold increase in the intracellular level of cyclic AMP (adenosine 3', 5'-monophosphate) reaggregated brain cell cultures derived from embryonic mouse brain. The cyclic AMP level of adult brain is increased by norepinephrine; however, embryonic mouse brain does not show a cyclic AMP response. The aggregate cultures thus demonstrate an event of differentiation very similar to that seen in vivo.