A chronological series of coordinated alterations in oocyte chromosome and microtubule disposition occur during oogenesis and oocyte maturation in the mammal. Timely transitions in meiotic spindle and cytoplasmic microtubules, due to modifications in both the assembly competence of the tubulin pool and nucleation capacity of centrosomes, underscore key nuclear events during the progressive stages of meiosis I and II. The regulation of these transitional states during meiosis is discussed with respect to hormonal influences imparted to the oocyte within the follicular microenvironment, and the possible ways in which environmental perturbations may result in defective chromosomal partitioning during meiosis. (Copyright (c) 1992 Elsevier Science Publishers B.V.)