Both nuclear transfer and intracytoplasmic sperm injection (ICSI) practice necessitates studies on the spatial relationship between the MII spindle and the first polar bodies (FPB). Although recent observations have shown that the FPB position does not predict accurately the location of the meiotic spindle in metaphase II oocytes of monkey, hamster, and human, detailed studies on FPB deviation and its affecting factors are lacking. Since polar bodies can be used for genetic testing and oocyte quality grading, their life span under different conditions should be studied. The timing of formation and degeneration and the position relative to the MII spindle of the FPB and the factors affecting FPB deviation and degeneration during in vivo and in vitro aging of both in vivo and in vitro matured mouse oocytes were investigated in this study. Mice of the Kun-ming breed were used, and the intact and degenerated FPB were identified through microscopic morphology in combination with propidium iodide (PI) exclusion test and the chromosomes visualized by Hoechst staining. Results are summarized as follows: (i) oocytes started FPB extrusion at 8 hr after the onset of in vivo or in vitro maturation, but the number of FPB reached maximum much later in vitro (14 hr of culture) than in vivo (10 hr post hCG). (ii) Some FPB began to degenerate before ovulation and around 70% became degenerated within 6 hr after maximal nuclear maturation both in vivo and in vitro; they disappeared faster during in vivo than in vitro aging but turned from intact to degenerated at a similar tempo. (iii) Some FPB began to deviate from the MII spindle 10 hr after hCG injection or in vitro culture and the distance between FPB and the spindle increased with time during both in vivo and in vitro aging. (iv) FPB deviated more slowly in the in vitro matured oocytes than in in vivo matured. (v) Denudation performed after FPB extrusion markedly enhanced its deviation. (vi) The perivitelline space (PVS) increased with time during maturation and aging in vivo and in vitro and the values of PVS and the percentages of FPB adjacent to the spindle were significantly negatively correlated. (vii) Cytochalasin B and colchicine had no effect on FPB deviation. (viii) None of the more than 3,500 FPBs observed was found to be dividing or have divided into two cells at any time points before or after ovulation or in vitro maturation. Our results were consistent with the possibility that the displacement of the FPB was a time- and PVS-dependent process, indicating that PVS would increase with time and its formation and enlargement would facilitate the lateral displacement of the degenerating FPB. Mol. Reprod. Dev. 69: 66–76, 2004. © 2004 Wiley-Liss, Inc.