Locomotor a c t i v i t y was r e c o r d e d from 11 female silver A m e r i c a n eels (Anguilla rostrata) r e c e i v i n g h o r m o n a l i n j e c t i o n s ' i n t e n d e d to induce maturation. Five eels treated with extracts of carp p i t u i t a r i e s became at least p a r t i a l l y mature, while 6 eels treated with o v i n e -l u t e i n i z i n g hormone remained u n a f f e c t e d (immature). Immature eels were more active n o c t u r n a l l y and showed peaks of a c t i v i t y at light-dark transitions. Maturing eels were equally active by day and by night, but r e m a i n e d r e s p o n s i v e to light-dark transitions. In constant darkness both groups i n c r e a s e d their activity at times c o r r e s p o n d i n g to expected light-off, suggesting an innate timing mechanism. When m a t u r i n g eels were subjected to constant light and p h o t o p e r i o d phase-shifts, they d i s p l a y e d t r a n s i e n t s which further e x e m p l i f i e d the use of a b i o l o g i c a l clock. The s i g n i f i c a n c e of these findings r e l a t i v e to the natural spawning behavior of eels is discussed.