Effects of kinetin (K), gibberellin A3 (GA3), and 2-(chloroethyl)-trimethylammonium chloride (CCC) on levels of alanine aminotransferase (GPT) and rates of protein synthesis were studied with both intact plants and isolated leaf segments of Lolium temulentum L. In intact plants CCC stimulated and CA3 reduced GPT activity, the effects bsing much greater in 8.h than in 16-h photoporiods. CCC showed maximum stimulatory effects at 10–2 M and K at 5 × 105 M. No effect of GA3 could be demonstrated with concentrations up to 10–4M. Both K and CCC retarded GPT decline in leaf sections, the latter without associated effects upon pigment breakdown. Cycloheximide was highly effective in reducing protein synthesis in leaf sections. A close correlation between rate of protein synthesis and GPT activity was found over an inhibitor concentration range from 10–6 to 10–4 M. The results are discussed in terms of possible methods of in vivo regulation of GPT activity.