An influence of early stimulation on sensitivity to acute stress in adulthood has been reported. The purpose of the present work was to determine the effect of exposure of male and female rats to three models of chronic stress (unpredictable stress, cold stress and handling) from day 2 to day 15 of life on behavioral and endocrine sensitivity to chronic stresses in adulthood. The chronic stresses applied in adulthood were a model of intermittent cold stress (daily 30-min sessions at -20 degrees C for 15 days) and the Katz's model of unpredictable chronic stress (15 days). Forced swim behavior and serum concentration of the stress-sensitive hormones, corticosterone and prolactin, were chosen to investigate stress sensitivity. It was found that all neonatal treatments stimulated body weight gain, did not cause infant mortality and did not affect forced swim behavior as adult. The repetitive exposure to cold stress in adulthood did not cause major impairment of forced swim behavior and did not affect basal levels of serum corticosterone and prolactin in either control or experimental rats. These findings support the view that repeated stressors can induce behavioral and endocrine adaptation in rats. The neonatal treatments did not affect this characteristic. The exposure of control rats to the unpredictable stress model severely impaired forced swim behavior and increased basal levels of serum corticosterone and prolactin. This observation conforms to the view that standard laboratory rats cannot adapt to unpredictable chronic stress. This has been reported to cause a behavioral depression syndrome comprising forced swim deficit and endocrine alterations.(ABSTRACT TRUNCATED AT 250 WORDS)