Aim: To determine whether maturation and subsequent blastocyst development of in vitro matured oocytes can be improved by in vivo follicle stimulating hormone (FSH) or human chorionic gonadotrophin (hCG) priming, using a mouse model.Experimental design: Five groups of oocytes were used: in vivo control, in vitro matured (IVM) control, IVM after 24 h in vivo priming with FSH, IVM after 48 h in vivo priming with FSH and IVM after 16 h in vivo priming with hCG. In vitro fertilization (IVF) was performed on all groups.Oocyte maturation, fertilization, blastocyst development rates and blastocyst cell numbers were assessed for all groups.Results: Significant improvement in oocyte maturation was observed in the two FSH priming groups compared with the IVM control group (P < 0.005 and P < 0.001, respectively). There were no significant differences in fertilization between all five groups. Blastocyst development was significantly higher in the in vivo control compared to the IVM groups (P < 0.001). No significant differences were observed in blastocyst cell numbers among all five groups.Conclusions: While FSH priming improves the maturation rate of IVM oocytes, FSH or hCG priming does not improve development to the blastocyst stage. (C) 2002 Elsevier Science Inc. All rights reserved.