The relative growth rate of pot-grown plants of Poa pratensis L. cv. Holt, origin 69s°N, was increased by 20–40% by photoperiod extension with low intensity incandescent light from 8 to 24 h at 9–21°C. The main increase occurred over the 14 to 18 h photoperiod range. The true photoperiodic nature of the response was demonstrated by the effectiveness of night interruption in stimulating growth. Fortnightly sprayings with gibberellic acid (GA3) (3 × 10-6 to 3 × 10-5M) mimicked all the effects of long days, whereas (2-chloroethyl)-trimethylammonium chloride (CCC) counteracted the effects of long days. Both growth substances exhibited pronounced interactions with photoperiod, GA3 being most effective in short days and CCC in long days. The growth stimulation, whether caused by long days or GA3, was exerted mainly through increases in individual and total leaf area. This was associated with a reduction in CO2, exchange rate and a parallel fall in specific leaf weight. Proportionally, however, the increase in leaf area was greater than the fall in CO2 exchange rate, resulting in a 38 to 118% increase in photosynthesis per leaf. No evidence was found of any direct and promotive effect of transition to long days on the CO2 exchange rate of already expanded leaves.