The aim of this study was to evaluate the influence of early educational quality on children's cortisol levels. It was hypothesised that the environmental stressors might load children's immature stress regulative systems thus affecting their diurnal cortisol levels. The study sample consisted of 146 preschool-aged children. Cortisol was measured during one day across five time points. The quality of learning environment was evaluated with the Learning Environment Assessment, focusing on indicators of psychological, physiological and social safety. The results revealed a typical daily rhythm in cortisol production characterised by higher levels in the morning on waking up followed by a decrease towards the afternoon and evening. In addition, the single early morning cortisol peak and the evening nadir indicated an average established function of the HPA-system. However, some children had cortisol pattern indicating clearly atypical HPA-activity. These children were sensitised to the effects of the learning environment. Low quality classroom arrangement and low quality team planning were associated with atypical cortisol patterns and with elevated cortisol levels. (Contains 3 tables and 4 figures.)