AbstractImmature rat pineal grafts were transplanted bilaterally to the anterior eye chamber of adult male rats. The transplants matured and became sympathetically innervated from the host iris and synthesized 5‐hydroxytryptamine as revealed by fluorescence histochemistry. Transplants were used to investigate whether the newly formed nerve terminals were functional and thus could induce a rhythm in pineal N‐acetyltransferase (NAT) enzyme activity and hydroxyindole‐O‐methyl transferase (HIOMT) enzyme activity. Thein situpineal glands from the recipient animals exhibited NAT and HIOMT levels and diurnal variations of enzyme activities no different from control rats without transplants. Sympathetically innervated transplants showed lower NAT enzyme activity than thein situpineals but did yield a highly significant increase in darkness NAT activity as compared to light NAT activity (20:1). Sympathetically denervated transplants showed a much smaller rise in darkness NAT enzyme activity as compared to light values (1.8:1) but the rhythm was still statistically significant (p<0.01). Sympathetically innervated transplants had a higher HIOMT activity in darkness than in light (2.7:1). The HIOMT activity of the sympathetically denervated grafts was invariably low. Protein contents of transplants were found to be significantly lower thanin situpineal protein content but failed to show a diurnal va