The effects of short-term food deprivation and photoperiod on plasma thyroid hormone levels of sea bass and sea bream were studied. Animals were acclimated under constant photoperiod regime (15L/9D) and feeding times (2 hr after light onset and 2 hr before light offset). Time-course studies involved monitoring plasma hormone levels every 4 hr throughout 1.5 24-hr cycles. Plasma 3,5, 3'-Triiodo-L-thyronine (T3) and L-thyroxine (T4) were assayed using a newly developed competitive enzyme immunoassay, utilizing acetylcholinesterase as a label of enzymatic tracers. Enzyme immunoassays had sensitivities of 1.25-0.02 and 62.5-0.2 ng/ml for T3 and T4, respectively, and reproducibilities of 3.7 and 5.6% intraassay variation for T3 and T4, respectively; interassay variations for T3 and T4 assays respectively were 1.6%, 11% and 6.6%, 8% for sea bass and sea bream plasma similar to RIA. In sea bass, 3 days of food deprivation resulted in depressed plasma T3 and T4, overriding significant diel variations seen during the second day of starvation. Sea bream displayed a slight decrease of T4 plasma levels while T3 levels remained constant for the whole sampling period. Both thyroidal systems responded to photoperiod with a significant increase in plasma T4 level at the time of light onset. In addition, sea bass also displayed increased T3 levels and decreases in both hormone levels coinciding with "lightoff." Data show different responses of the sea bass and sea bream thyroidal systems to both nutritional state and photoperiod in that the latter state is influenced by the former. Data suggest plasma thyroid levels can be used as a rapid indicator of nutritional status.