1. Mild stress plus mild pain (solvent injection) applied daily to neonatal mice induces hormonal, behavioural and metabolic changes perduring in the adult life. 2. We investigated whether daily mild stress to neonatal mice induces also long-term defined changes of immune response, and whether immune changes are prevented through repeated administration of the opioid antagonist naloxone. 3. Mild stress plus solvent injection administered from birth to the 21st postnatal day causes not only behavioural and metabolic changes, but also long-term (up to 110 days of life) splenocytes modifications, consisting in: increased release of the Th-1 type cytokines interleukin-2 (IL-2) (from an average of 346 to 788 pg ml(-1)), interferon-gamma (from 1770 to 3942) and tumour necrosis factor-alpha (from 760 to 1241); decreased release of the Th-2 type cytokines IL-4 (from 49.1 to 28.4) and IL-10 (from 1508 to 877). Moreover, enhanced natural killer-cell activity; enhanced proliferative splenocytes properties in resting conditions and following phytohemoagglutinin and concanavalin-A stimulation are observed. Immunological, behavioural and metabolic changes are prevented by the opioid antagonist (-)naloxone (1 mg kg(-1) per day s.c., administered instead of solvent) but not by the biologically inactive enantiomorph (+)naloxone. 4. In conclusion, endogenous opioid systems sensitive to naloxone are involved in long-lasting enhancement of the Th-1 type cytokines and cell-mediated immunological response caused by repeated mild stress administered postnatally.