Gas exchange in leaves of different age was studied in silver birch (Betula pendula) and Karelian birch (Betula pendula var. carelica) seedlings during intensive shoot growth under optimal external conditions and adequate soil water content. Average photosynthesis in Karelian birch (KB) and silver birch (SB) was 11.39 and 12.30 μmol•m.2•sec.1, respectively, the maximums being 28.3 and 21.8 μmol•m.2•sec.1. The differences in transpiration were 1.97 and 2.42 μmol•m.2•sec.1 in KB and SB, respectively. We estimated stomatal and mesophyll conductance, CO2 diffusion resistance, and transpiration efficiency. Where soil moisture was sufficient, the two birch forms differed reliably in terms of stomatal and mesophyll conductance means. Stomatal conductance in KB leaves was on average 25 % lower than in SB. Photosynthesis in KB remained at a level similar to that in SB owing to higher mesophyll conductance. Transpiration efficiency was 32 % higher in KB than in SB. Photosynthesis in the leaves ceasing to grow was less intensive than in the fully developed leaves because of lower mesophyll conductance, as well as stomatal conductance and chlorophyll content.