Bioluminescence in the midshipman fish, Porichthys notatus from the Santa Barbara coastal region, was quantified from onset through the first two years of life. Maximum light emission was 2.5 x 109 photons s-1 upon leaving the nest and reached 2.0 x 1010 photons s-1 within the first year. These intensities may be sufficient for counterillumination in moon or starlight over most of the depth range of the fish. The bioluminescence of juveniles recently detached from the nest was depleted by multiple topical applications of a dilute noradrenalin solution. A luciferin-free diet also exhausted luminescence in 10-18 months. Bioluminescence was restored within 24 h after feeding depleted fish with dried specimens of the bioluminescent marine ostracod Vargula hilgendorfii, and light emission capacity was correlated with the amount consumed. Predation by second year fish (18 months) upon juvenile P. notatus (3 months) or upon live V. tsujii also restored luminescence. After restoration, luminescence gradually disappeared within several months. Consumption of luciferin-containing organisms by already competent fish did not increase light intensity. Juvenile P. notatus from the Santa Barbara coastal region require exogenous luciferin to remain luminescent.