The present study was conducted to elucidate the effects of ACTH application during pregnancy on the female offsprings' endocrine status and behavior in a precocial animal, the guinea pig. We, therefore, studied daughters whose mothers were treated three times with either ACTH-depot or placebo during the sensitive phase of central nervous sexual differentiation of the foetus. From their 41st through their 80th day of age the spontaneous behavior of the daughters was recorded in their home cages, and concentrations of serum cortisol were determined from the 20th through the 90th day of age every 14 days. Daughters whose mothers had received ACTH displayed significantly lower amounts of a defensive aggressive behavior pattern, the urine spray, and they had higher cortisol concentrations than daughters whose mothers had received placebo. Thus, the number of species that shows consequences of an ACTH treatment during pregnancy for the offsprings' endocrine status and behavior is extended to include a precocial animal, the guinea pig.