The present work aims to study the effects of estrogen or endocrine disrupters (EDs) on the dynamic changes in intracellular Ca2+ concentration of mouse immature oocytes (IOs) loaded with Ca2+-sensitive dye Fura-2 using an image analyzer. The majority of IOs isolated from the ovary exhibited spontaneous Ca2+ oscillations at regular intervals. Entry of external Ca probably through gap junctions, contributes to Ca2+ oscillations since they were reversibly inhibited by removing Ca2+ from the bathing medium or by the application of a gap-junction inhibitor carbenoxolone (CBX, 30 muM). Both 17beta-estradiol (E2) and E2-BSA, a membrane impermeable estrogen, shortened the duration of Ca2+ oscillations in a dose-dependent manner (1-1000 nM), and produced an irregular pattern of the oscillations, strongly suggesting that E2 acts on the plasma membrane of the oocyte. For bisphenol A (BPA), one of the estrogen-mimicking EDs, a 10,000-fold higher concentration (100 muM) was necessary to exert similar inhibitory action to that of E2. (C) 2004 Elsevier Inc. All rights reserved.