The effect of nitrogen starvation on photosynthetic pigments and energy coupling was compared in Synechococcus sp. (Cyanophyta) strains originating from oceanic (oligotrophic) or coastal (eutrophic) marine environments. A survey indicated that those of oceanic or subtropical origin retained a greater fraction (55 to 98 %) of their major phycobiliprotein during a 24 h nitrogen starvation period compared to coastal strains (30 to 44 % ) For 3 strains studied in detail, nitrogen starvation caused a significant (> 85 ':L) loss of phycoerythrin from Synechococcussp. WH8018 after 24 h, but only a minor loss (< 25 %) from Synechococcus sp. WH7803 or WH8103 after 3 d starvation. All 3 strains exhibited reduced gross oxygen evolution dunng the first 24 h of starvation, however, indicating a reduction in energy transfer from phycoerythrin to the electron transport chain. Changes during starvation in the in vivo fluorescence excitation and emission spectra indicated a small degree of uncoupling of phycoerythrin from allophycocyanin in Synechococcus sp. WH7803 and MrH8103 but not Synechococcus sp. WH8018. In neither case could it account for the measured loss in photosynthetic efficiency, however. The unusual nature of phycobiliprotein regulation in oceanic strains may reflect an adaptation to episodic (few days time scale) inputs of limiting nutnents to oligotrophic surface waters, thus providing a mechanism for more rapid rejuvenation.