An automatic system has been developed to measure the variations in the rate of shoot elongation of tree seedlings. A camera equipped with 50 mm macro-objective lens, with automatic flash and winder is attached to a motor-operated rotatable stand. The camera is connected to an intervalometer enabling exposures of various seedlings placed around the stand to be taken in sequence at a given time interval. The time interval between the exposures of each seedling is adjustable between 5 min and one week. A paper scale with 0.25 mm rulings is placed behind each shoot. The negatives are projected onto a screen with a X 10 enlargement and the shoot lengths measured. The results show that the resolution of the device is +/- 0.05 mm. The system was tested under field conditions in June 1979 at the Forestry Field Station of the University of Helsinki with containerized Scots pine (Pinus sylvestris L.) seedlings. The daily course of the shoot elongation rate closely followed the variations in ambient temperature. The system can be applied to the measurement of other plant species.