Comparison of effects of synthetic ovine corticotropin releasing factor (oCRF), Arginine-Vasopressin (AVP) and the combination of both peptides have been tried in adult and 7-days-old guinea-pigs. On plasmas collected 15 min after interscapulary injection, cortisol, aldosterone and ACTH were measured. The different circulating forms of ACTH were isolated by Sephadex G50 column chromatography, with 1% formic acid and measured by radioimmunoassay. Thus, in the guinea-pig plasma, we detected three immunoreactive forms of ACTH: a "big" molecular form (Mr greater than 20000), an "intermediate" (Mr = 9500) and a "little" ACTH form (Mr = 4500) which was eluted in the same fractions as human 1-39 ACTH. In adult guinea-pigs, CRF increased total ACTH and the "intermediate" form and also plasma cortisol concentrations whereas AVP remained without significant effect excepted a rise in cortisol levels. Injected together, CRF and AVP enhanced plasma concentrations of total ACTH, of the three circulating forms and of cortisol. In 7-days-old guinea-pigs, both CRF and AVP increased plasma concentrations of total, of "intermediate" ACTH and of cortisol and aldosterone whereas the combination of both peptides enhanced dramatically plasma concentration of total ACTH suggesting a magnifying effect of AVP on CRF activity still more efficient in young than in adult guinea-pigs.