The retention of photosystems I and II and or RuDP carboxylase activity in chloroplasts isolated from the first leaves of Victory oat (Avena sativa L.) seedlings was followed as the chloroplasts senesced in darkness. Both photosystems (PS) I and II retained their full activity after 3 days at 1°C, while even after 7 days at 1°C around 80% of the activity was still present. After 3 days at 25°C, PS I lost only 20% and PS II 50% of the initial activity. Acid pH increased the rate of decay of both systems, PS II falling almost to zero after 3 days at pH 3.5 (at 25°C). The preparations were almost bacteria-free, and addition of antibiotics not only did not improve their stability, but accelerated the rates of loss of photosynthetic activity. This is held to indicate that the enzymes are undergoing some turnover even in isolated chloroplasts. If the leaves were allowed to senesce in the dark first and the chloroplasts then isolated, their photosynthetic activities had greatly decreased, showing that senescence is more rapid in situ than in isolation. Under these conditions PS I decayed more rapidly than PS II. Ribulosediphosphate carboxylase, as measured by CO2 fixation, declined more rapidly than the photosystems, though the addition of kinetin and indole-3-acetic acid somewhat decreased the rate of loss, at least for the first 24 h. When the intact (detached) leaves were held in the dark, the rate of oxygen evolution declined rapidly, but in monochromatic blue light (450 nm) at 25°C about 30% of the initial rate was retained after 72 h.