Changes in hexokinase particulate and soluble isozyme composition and activities in leaves of 65- and 115-d-old tobacco plants were determined by ion exchange chromatography on DEAE cellulose. During plant ageing, the activities of glucose and of fructose phosphorylating isozymes of particulate hexokinase decreased to 9.9 and 9.2 % of initial value, respectively. The activity of soluble hexokinase decreased to a lesser extent: that of glucose phosphorylating isozyme to 49.8 % and of fructose phosphorylating isozyme to 37.8 %. The activity of soluble fructokinase isozyme dropped to 34.8 %. Thus also the ratio of particulate and soluble isozymes was dependent on the age of leaf tissue.