A simple mathematical model is described for simulating the variation in the growth rate of shoots and needles, and the rate of stem diameter growth in 28-year-old Scots pine (Pinus sylvestris L.) trees. It is assumed that growth rate is affected by environment and by the internal state of the plant. Temperature is the only driving variable in the model, having an effect (1) on the rate of physiological development and (2) on the growth rate at any stage of development. The effect of the physiological stage of development on the growth rate is brought into the model by means of the concept of inherent growth rhythm. Field measurements were carried out in Central Finland in 1974 and 1975. The percentage of variance explained by the model was between 68 and 93 for shoot and needle elongation, and between 50 and 60 for radial growth. The seasonal variation in the growth phases of the studied components followed a similar pattern during both of the years of the study. However, depending on the climatic conditions prevailing in early spring, the timing of the phases varied by 1-2 weeks. No major variation was observed between the upper and lower parts of the crown in the timing of the growth phases. The magnitude of shoot growth in the upper part of the crown was about five times greater than that in the lowest branches. The model can be utilized in assessing the magnitude of the simultaneous or rapid response to environmental factors other than temperature, which in nonstressed conditions seems to have only slowly acting effects on growth. Forest Sci. 28:573-581.