A study was carried out to determine the extent of the variation in growth rate between several selected progenies of Scots pine ( Pinus sylvestris L.) and the physiological characteristics underlying such variation, using growth analysis and direct measurement of photosynthetic rates. Differences in relative growth rate (RGR) could be demonstrated for first and second year seedling progenies, and these largely reflected differences in net assimilation rate (NAR). The relative performance of the progenies in different experiments was not always the same, although the highest RGR was shown by the same progeny in both firstyear and second-year seedlings. For most progenies the observations were extended to 14-year-old trees growing in field trials established by the Forestry Commission. Comparisons of the relative height growth rates (RHGR) of these trees showed no significant differences between the progenies, apparently because the growth rates were limited by the site conditions. Thus, the size differences at the end of the fourteenth year probably still reflected differences in RGR which occurred during the seedling stages in the nursery. The significance of these observations for the assessment of performance in tree breeding programmes is discussed.