The prolonged effects of neonatal SRIH-14 treatment on pituitary ACTH cells were investigated. Neonatal female rats were injected subcutaneously with SRIH (20 microg/100g b.w.) every 12 hours for five consecutive days (3rd-7th day of life). Groups of rats were then killed at the juvenile (16th day), peripubertal (38th day) or adult (80th day) stage. ACTH cells were visualized using the peroxidase-antiperoxidase immunocytochemical procedure. Morphometry and stereology were used to evaluate the ACTH-immunoreactive cell volume and volume density. The histological and immunocytochemical characteristics of ACTH cells in neonatally treated females were changed in all examined periods. Thus, SRIH-14 induced significant (p<0.05) decreases of ACTH cell volume in juvenile, peripubertal and adult rats by 26%, 39% and 14%, respectively, in comparison to the corresponding controls. The volume density of ACTH cells was also diminished (by 31%; p<0.05) at the juvenile stage in comparison with the corresponding controls. In peripubertal and adult rats, the volume densities of ACTH cells were somewhat lower (17% and 14%, respectively), but the decreases did not reach statistical significance. These findings suggest that neonatal treatment with SRIH-14 exerts a marked prolonged inhibitory effect on ACTH cell morphology extending to the adult period of life.