Summary Scots pine ( Pinus sylvestris L.) seedlings were grown under controlled growth conditions with three relative addition rates (2, 4 and 6 % RAR per day) of potassium. The relative growth rate (R G ) of the seedlings, and the potassium, nitrogen, phosphorus and free polyamine concentrations of the needles and roots were measured during the growth period. Visible symptoms of potassium deficiency developed in the needles at the lowest potassium level after 6 weeks. Although the needle potassium concentrations and the relative growth rate correlated positively, the decrease in potassium from the highest K level (6 % RAR) to the medium one (4 % RAR) affected only slightly the relative growth rate per day, which decreased from RG 5.8 % to 5.5 %. At the same time putrescine accumulated in needle and root tissues. A clear decrease in the relative growth rate from RG 5.5 % at the 4 % RAR level to RC, 4.7 % at the lowest 2 % RAR level coincided with the high putrescine concentrations and low spermidine and spermine concentrations of the needles at the 2 % RAR level. The spermidine and spermine concentrations of the roots were lower than in the needles and at about the same level in all the treatments. These results show that the putrescine concentrations indicate the potassium status of the seedlings and that spermidine and spermine may be involved in changes of the growth rate of the Scots pine seedlings under varying potassium nutrition.