Fluorescence decay measurements using a mode-locked argon laser and single photon time-correlated detection, help us understand energy migration in photosynthesis. For intact spinach chloroplasts we find multiple exponential decays. There is some hint of a very fast component (<100 ps), but our data reveal little about it. We would expect this to be due to "photosystem I units." In addition we find two components at longer times, both of which lengthen as conditions are changed from "open" reaction centers (0.1 and 0.7 ns) to "closed" reac-tion centers (1 and 2 ns). This may reflect two kinds of organization for "photosystem II units."