Male rats have been reared as litters of 2 pups (group A) or 12 pups (group B) during the first 3 weeks of life. Males of group A exhibited higher body weights from day 10 to day 430, reached puberty at earlier age and showed lower emotionality as well as diminished learning capability and decreased memory capacity in adulthood as compared to males of group B. Furthermore, adult males of group A developed an increased ratio of insulin to glucose serum concentration at 90 minutes after glucose administration.