Activity changes in alanine aminotransferase (glutamate-pyruvate transaminase, GPT) in exapanded fifth leaves of Lolium lemulentum L. were studied withn plants exposed to 8-h and 16-h photopepriods at 20°C. Two phases were detected. The first was daylength independent and characterized by a rising activity either on a fresh-weight or protein basis, whilst the second phase consisited of a rapid fall to a much lower activity during the course of a few days. This effect was promoted by 16-h/20°C conditions. Using variable exposure to long days and light-break treatments it was shown that the GPT deporessions did not relate directly to floral induction but was it response to increased incident light energy. Comparison with asparate aminostransferase, acid phosphatase, malate dehydrogenase, and peroxidase suggested that GPT activity could be used as a sensitive indicator senescence.