Platymonas subcordiformis (UTEX 171) was cultured axenically for 4 d in constant light in a nitrate-containing medium and harvested in the log-phase of cell division. Ceils were resuspended in artificial sea water without nutrients and either kept in constant light or placed in constant darkness. High-performance liquid chromatography was used to measure the free amino acid pools of the ceils and to determine rates of net entry of each of a mixture of 18 amino acids at daily intervals for 5 d. Free amino acid pools decreased both in light and darkness in the absence of a nutrient supply. The influx of amino acids in cells maintained in the light increased selectively. Comparison of the rate of entry of 14C_labele d glycine and net disappearance of glycine from the medium indicated extrusion of non-volatile labeled carbon that did not interact with reagents specific for amine groups. Light was required for synthesis of additional transporter protein which was apparently responsible for increased influx in cells maintained in the light. This response was blocked in the presence of cycloheximide. Cells maintained in the dark for prolonged periods retained the capacity to respond to light by synthesis of new transporter protein. Analysis of incorporation of amino acids into macromolecules indicated that both the overall rate and the pattern of amino acid incorporation were modified in the light. Analysis of the kinetics of glycine entry at a series of temperatures indicated that the concentration of glycine at which entry is half the maximum rate is approximately 2.7/IM at the cell surface.