New strategies were proposed to improve the developmental competence of calf oocytes through in vitro technologies. Cumulus-oocyte complexes were first prematured for 24 h in the presence of meiosis inhibitors. Both Roscovitine alone (50 microM) or in combination with Butyrolactone-I (12.5 microM Rosco+6.25 microM BL-I) prevented the progression of meiosis. Their effect on nuclear maturation was reversible after a further 17 or 24 h maturation step. However, a dramatic decrease in embryo development was observed after fertilization (abattoir oocytes: 4-9% blastocyst rate versus 14-17% for control embryos). Similar results were obtained with oocytes collected by Ovum Pick Up from living donors. No pregnancy was obtained after single transfer of two blastocysts obtained from prematured oocytes (0/2 versus 4/12 for control embryos). Adding low concentrations (1, 3 or 10 microM) of follicular fluid-meiosis activating sterol (FF-MAS) during the maturation step had a beneficial effect on nuclear maturation (73-86% metaphase II versus 58% for control oocytes). However, subsequent embryo development was not improved. Enriching the maturation medium, namely with hormones, growth factors and precursors of glutathione, induced a sixfold increase in glutathione in the oocyte and had a beneficial effect on embryo development (38% increase in blastocyst rate). In conclusion, in opposition to the results reported with adult oocytes, prematuring calf oocytes had a negative impact on their developmental potential. Although FF-MAS improved nuclear maturation, its addition in the maturation medium did not increase embryo development. However, enriching the maturation medium had a positive effect on embryo development, indicating that cytoplasmic maturation was improved.