Int. J. Environ. Res. Public Health 2010, 7 2034 facilitate blood collection via cardiac puncture for estradiol-17 (E2) measurement by RIA. Ovulation was induced in the remaining mice at 48 hours post PMSG with human chorionic gonadotropin (hCG) and immediately caged with adult males for mating. Five plugged female mice/group were sacrificed for the determination of ovulation rate. The remaining mated mice were sacrificed about 26 hours post hCG, ovaries excised and weighed and embryos harvested and cultured in Whitten’s medium (WM) supplemented with CZBt. In Experiments 2 and 3, (N = 10/Experiment) folliculogenesis and ovulation were induced in untreated mice followed by mating. Recovered embryos were either exposed continuously (Experiment 2) or intermittently (Experiment 3) to bioavailable HU (18 g HU/mL of WM + CZBt) or WM + CZBt only (control). Treated mice sustained decreased ovarian wt, ovulation rate and circulating E2 compared with controls (P < 0.05). Fewer embryos retrieved from HU-treated mice developed to blastocyst stage (32%) compared with those from controls (60%; P < 0.05). Furthermore, continuous or intermittent in vitro exposures of embryos to HU also resulted in reduced development to blastocyst stage (continuous HU, 9 vs. control, 63%; P < 0.05; intermittent HU, 20 vs. control, 62%; P < 0.05) with embryos exposed continuously to HU in vitro fairing worse. Even though HU is well tolerated, our data suggest that it compromises folliculogenesis and the ability of generated embryos to develop. Therefore, designed studies with larger numbers of patients receiving HU during pregnancy, with longer follow-up of exposed children and more careful assessment of embryo/fetotoxic effects, are required before this agent can be promoted as safe in pregnancy. Keywords: ovarian weight; estradiol-17 ovulation rate; 2-cell embryo; blastocyst; hydroxyurea 1. Introduction Hydroxyurea (HU) is a low molecular weight nonalkylating myelosuppressive agent that impairs DNA synthesis through the inhibition of ribonucleotide reductase [1]. This drug is often used to supplement phlebotomy for patients with polycythemia vera (PV; [2]) and to reduce significantly the incidence of recurrent thrombosis in patients with essential thrombocythemia who are at high risk [3]. Even though the therapeutic use of HU may be declining in patients with chronic myeloid leukemia [4], its use for the treatment of patients with sickle cell disease (SCD; [5]) is gaining acceptance. In the latter patients, HU alleviates the classical symptoms of the SCD not by its anti- mitotic effect but by provoking increased production of fetal hemoglobin which has a higher oxygen carrying capacity than sickle cell hemoglobin. Currently, HU is used investigatively for the treatment of human immunodeficiency virus (HIV) infection [6,7]. In the latter treatment, HU does not have a direct anti-virus activity; rather, it inhibits the cellular enzyme ribonucleotide reductase and as a consequence, reduces deoxynucleotide triphosphates (dNTPs) that are necessary for DNA synthesis.