In this study, melatonin (MEL) and thyroxine (T(4)) concentrations were measured during larval and postlarval development of gilthead sea bream Sparus auratus Hormones were measured in whole bodies of larvae or the head and trunk of postlarvae after 67 days of exposure to constant light, 24L:0D, constant darkness, 0L:24D or 12L:12D and in the plasma of 6 month juveniles kept under the 12L:12D, 0L:24D and 24L:0D regimes. High MEL concentrations in larvae suggested a distinct role of MEL in early organogenesis and development of S. auratus. In larvae, the gastro-intestinal tract seemed to be an important extrapineal and extraretinal source of MEL. No endogenous rhythm of MEL synthesis was demonstrated in 67 day larvae; however, in 6 month juveniles, it was evident. At early ontogenesis of S. auratus, the role of MEL is probably related mostly to the control of development and protection against free radicals, whereas its action as a time-keeping molecule develops later. The increase in T(4) concentration during the S. auratus larva-juvenile transition, i.e. between 50 and 70 days post-hatch, which was observed concurrently with the decrease of MEL concentration, may suggest an inverse relationship between T(4) and MEL.