To determine possible causes of reported differences between developmental competence of oocytes isolated from prepubertal (10- to 14-week-old calves) and adult cows, three parameters were analysed, comparatively, during in vitro maturation (IVM): (1) oocyte diameter, (2) oocyte energy metabolism, and (3) protein synthesis of oocytes and cumulus cells. Cumulus-oocyte complexes were isolated from follicles of 3-5 mm in diameter in both age groups. Mean oocyte diameter was smaller (P < 0.02) in calves than in cows (118.04 +/- 1.15 versus 122.83 +/- 0.74 microns). During the first 3 hr of IVM, calf oocytes metabolised glutamine and pyruvate at lower rates than adult oocytes, but after 24 hr of culture, both molecules were metabolised at the same rate as for adult oocytes. A significant decrease in protein synthesis, as measured by [35S]methionine and [35S]cysteine incorporation was recorded after 9 hr of IVM in calf oocytes, while in adult oocytes a significant decrease in protein synthesis was detected only after 24 hr. After the first 3 hr of maturation, proteins of 130, 26, and 24 kDa were more abundant in adult than in calf oocytes, while a protein of 55 kDa was more visible in calf than in adult oocytes. At the same time, among proteins newly synthesised by cumulus cells, molecules of 405, 146, 101, and 77 kDa were more abundant in adults than in calves. In conclusion, calf oocytes and cumulus cells showed several differences when compared with their adult counterparts, which are consistent with their reported lower developmental competence.