Fluctuating asymmetry (FA) refers to small, non-directional deviations from perfect bilateral symmetry in morphological characters. Individuals with low FA presumably either developed in a relatively stable environment and/or were better able to buffer against developmental stressors. The present study investigated the effects of seasonal factors measured by day length and ambient temperature manipulations on the development of bilateral characters and concomitant changes in stress responses. Siberian hamsters were exposed to either long days (16 h of light per day) or short days (8 h of light per day) combined with either standard temperatures (21+/-2 degrees C) or low temperatures (8+/-2 degrees C) on the day of birth until weaning. Cortisol concentrations at baseline and following acute restraint stress, and FA values were measured in adulthood. Females reared in winter-like conditions with short day lengths and low temperatures had low FA and low cortisol concentrations following restraint stress compared to other females. Females reared in long day lengths and standard temperatures had the highest rate of increase in cortisol concentrations after restraint among other female groups. No group effects were observed in males regarding day length and temperature manipulations. Baseline and post-restraint cortisol concentrations were higher in females than males for all groups except in animals reared in short day lengths and low temperatures. Our results suggest that winter-like conditions during neonatal period evoke hyposensitivity to stress in adult females and this blunted response to stress is a key factor in achieving ideal growth patterns.