The kinetic response of photosystem II (PS II) photochemistry in Spirulina platensis (Norstedt M2) to high salinity (0.75 M NaCl) was found to consist of two phases. The first phase, which was independent of light, was characterized by a rapid decrease (15-50%) in the maximal efficiency of PS II photochemistry (F(v)/F(m), the efficiency of excitation energy capture by open PS II reaction centres (F(v)'/F(m)'), photochemical quenching (q(p)) and the quantum yield of PS II electron transport (Φ(PS II)) in the first 15 min, followed by a recovery up to about 80-92% of their initial levels within the next 2 h. The second phase took place after 4 h, in which further decline in above parameters occurred. Such a decline levels after 12 h. At the same time, non-photochemical quenching (q(N)) and Q(B)-non-reducing PS II reaction centres increased significantly in the first 15 min and then recovered to the initial level during the first phase but increased again in the light in the second phase. The changes in the probability of electron transfer beyond Q(A) (Ψ(o) and the yield of electron transport beyond Q(A) (φ(Eo), the absorption flux (ABS/RC) and the trapping flux (TR(o)/RC) per PS II reaction centre also displayed two different phases. The causes responsible for the decreased quantum yield of PS II electron transport during the two phases are discussed.