|Abstract:||The effect of growth light intensity on the photosynthetic apparatus of pea thylakoid membranes was investigated. Peas were grown either in a growth chamber at light intensities that ranged from 8 to 1050 $\mu$E/m$\sp2$ $\cdot$ s, or outside under natural sunlight. In thylakoid membranes the amounts of the following photosynthetic components were determined: active and inactive photosystem II, photosystem I, cytochrome b/f, high-potential cytochrome b-559, the rate of uncoupled electron transport, and the ratio of chlorophyll a to b. In leaves the amounts of the photosynthetic components per leaf area, the fresh weight per leaf area, the rate of electron transport and the light compensation point were determined. The maximum difference in the concentrations of the photosynthetic components in peas grown above the light compensation point (20 to 40 $\mu$E/m$\sp2$ $\cdot$ s) was about 30% in thylakoids isolated from plants grown over a ten-fold range in light intensity, 100 to 1050 $\mu$E/m$\sp2$ $\cdot$ s. Peas grown under natural sunlight were virtually indistinguishable from plants grown in growth chambers at the higher light intensities. On a leaf area basis, over the same growth light regime, the maximum difference in the concentration of the photosynthetic components was also about 30%. For peas grown at 1050 $\mu$E/m$\sp2$ $\cdot$ s we found the concentrations of active photosystem II, photosystem I, and cytochrome b/f were about 2.1 mmol/mol chlorophyll. There was an additional 20 to 33% of photosystem II complexes that were inactive. In our view, these data do not constitute convincing evidence that the stoichiometries of the electron transport components in the thylakoid membrane, the size of the light-harvesting system serving the reaction centers, or the concentration of the photosynthetic components per leaf area, are regulated in response to different growth light intensities. The concept that emerges from this work is of a relatively fixed photosynthetic apparatus in thylakoid membranes of peas grown above the light compensation point.