The effects of hydrostatic pressure on the excited state reactions of the photosynthetic system of cyanobacteria were studied with the use of stationary and dynamic fluorescence spectroscopy. When the cells were excited with blue light (442 nm), hydrostatic pressure promoted a large increase in the fluorescence emission of the phycobilisomes (PBS). When PBS were excited at its main absorption band (565 nm), the shoulder originated from photosystem II (PSII) emission (F685) disappeared under 2.4 kbar compression suggesting suppression of the energy transfer from PBS to PSII. At atmospheric pressure, the excited state decay was complex due to energy transfer processes and the best fit to the data consisted of a broad Lorentzian distribution of short lifetimes. At 2.4 kbar, the decay data changed to a narrower distribution of longer lifetimes, confirming the pressure-induced suppression of the energy transfer between the PBS and PSII. When the cells were excited with blue light, the decay at atmospheric pressure was even more complex and the best fit to the data consisted of a two-component Lorentzian distribution of short lifetimes. Under compression, the broad distribution of lifetimes, mostly attributed to PSII, disappeared and gave rise to the appearance of a narrow distribution characteristic of the phycobilisomes (PBS). The emission of PSI also became pronounced at 2.4 kbar at room temperature. A decrease in temperature from 20o to -10o C at 2.4 kbar promoted a further increase in the fluorescence emission from PSI to a level comparable to that one obtained at 4 K and atmospheric pressure. On the other hand, when the temperature was decreased under pressure, the PBS emission diminished to very low values at blue or green excitation, suggesting disassembly of the phycobiliprotein subunits from the complex followed by nonspecific quenching.