Abstract The ontogenetic changes in stomatal size, frequency and conductance (g s ) on abaxial and adaxial leaf surfaces of sunflower plants ( Helianthus annuus L. Russian Mammoth) were examined under controlled environmental conditions. The stomatal frequency on the adaxial and abaxial leaf surfaces decreased with leaf ontogeny and insertion level. The ratio of adaxial to abaxial stomatal frequency did not change with leaf ontogeny and insertion level, and 42–44% of total stomata was apportioned to the adaxial surface. Ontogenetic changes in stomatal pore length were detected and increased with ontogenesis. The stomatal length of both leaf surfaces had linear relationships with leaf area. Ontogenetic changes in g s were similar between the two surfaces. However the adaxial g s was lower than abaxial g s in leaves of higher insertion levels. Conductance had a linear relationship with width x frequency but not with pore area.