Net photosynthesis (Pn) was measured on alfalfa, Medicago saliva L., leaves of different ages from a growth chamber and from normal and thinned field plots. Pn declined with leaf age after full expansion. The mean Pn for young leaves was 52 mg CO2 dm$#x2212;2 hr−1 when grown in the field and 35 mg CO2 dm−2 hr−1 when grown in the growth chamber. However, when photosynthesis was expressed on a dry weight basis this difference no longer existed. Leaves remained alive more than 60 days in the growth chamber but lived only half as long in the field. Thinning field plots delayed senescence.Specific leaf weight (SLW = leaf dry weight/leaf area) was influenced by the light intensity and its change within the environment. SLW for young leaves was 5 mg cm2 when grown in the field and 3 mg cm−2 when grown in the growth chamber. SLW increased with leaf age in the growth chamber but decreased with leaf age in unthinned field plots.