Effect of three Zn2+ concentrations, i.e. 0.075 (cl), 7.50 (c2) and 37.5 (c3) jiM, on rice seedlings was studied at three stages, i.e. 1, 14 and 21 d after transplantation. Typical deficiency symptoms were observed in both solution and sand cultures of cl and c2, but the effects were more pronounced in the solution culture. The c3 concentration was toxic. There was marked reduction in growth, chlorophyll (Chl) contents (particularly Chl b), Hill reaction activity, photophosphorylation rate (particularly non-cyclic photophosphorylation), thylakoid phosphorylation, and i'‘C02-fixation at the cl concentration. However, a similar reduction was also observed in thylakoid phosphorylation at the c3 concentration. Hence the optimum zinc concentration in the nutrient medium lied between c2 and c3 Zn2+. By regression the theoretical optimum Zn concentration was calculated as 19.20 pM (1.28 mg kg'i) Zn2+. Partitioning of ^'^C-photosynthates indicated reduced allocation to sugar and starch fractions and increased fřee amino acids concentration at the cl concentration and vice-versa at c2 and c3.