We investigated changes in photochemical activity and cold hardiness of detached needles of three clones of Picea abies (L.) Karst. by measuring variable chlorophyll fluorescence (F(v)/F(m)), before and after artificial freezing, from September to June. Photochemical activity varied considerably during the study, but only minor differences in photochemical activity among the clones were observed before freezing. Photochemical activity was high during early fall and then declined from November until April. Photochemical activity was at a minimum in April and then increased quickly to high values in May. During the period from late September to October, and also during the winter, differences in F(v)/F(m) ratios after artificial freezing to below -10 degrees C were observed among clones, indicating clonal differences in cold hardiness and hardiness development. The clone having an average height of 2.3 m after 11 years showed consistently lower cold hardiness than clones that had reached average heights of 4.0 and 5.0 m. There were also differences in the temperature requirement for bud flushing among clones.