The recently passed Food Quality Protection Act of 1996 requires the U.S. EPA to implement screening strategies for endocrine active compounds (EACs) within the next 2 years. Interpreting results from screening tests is complicated by the absence of traditional dietary rodent bioassay data with model estrogenic compounds such as 17 beta-estradiol. Thus, a 90-day/one-generation reproduction study with 17 beta-estradiol was designed to: (1) provide such baseline data; (2) set dose levels for multigeneration reproduction and combined chronic toxicity/oncogenicity studies; and (3) evaluate various mechanistic/biochemical endpoints for inclusion in these follow-up studies. The current article describes the effects of dietary administration of 0, 0.05, 2.5, 10, and 50 ppm 17 beta-estradiol on the serum hormone concentrations and estrous cyclicity of female Crl:CD BR rats and evaluates a sampling strategy for measuring serum hormone levels in cycling female rats. Serum hormones were measured at three time points during a 90-day dietary exposure (1 week, 28 days, and 90 days) and in the F1 generation rats on postnatal day 98. Over the course of the 90-day feeding study for the P1 generation and from postnatal days 21 to 98 for the F1 generation, the estrous cycle was monitored daily in 10 rats/group. In P1 generation rats, dietary administration of 2.5, 10, and 50 ppm 17 beta-estradiol produced a dose-dependent increase in serum estradiol (E2) concentrations at all time points. In contrast, administration of 0.05, 2.5, 10, and 50 ppm 17 beta-estradiol produced a dose-dependent decrease in serum progesterone (P4) concentrations on test day 90, which correlated with an absence of corpora lutea and ovarian atrophy. At 10 and 50 ppm 17 beta-estradiol, serum luteinizing hormone (LH) concentrations were consistently decreased at all time points and were decreased at 2.5 ppm on test day 90. Serum prolactin (PRL) concentrations were increased at 50 ppm 17 beta-estradiol on test day 90. Serum follicle stimulating hormone (FSH) concentrations were either similar to the control levels or minimally changed at all time points. No F1 generation rats were produced at 10 or 50 ppm 17 beta-estradiol. In F1 generation rats, serum E2 concentrations were increased and P4 concentrations were decreased at a dietary concentration of 2.5 ppm 17 beta-estradiol, while serum concentrations of LH, FSH, and PRL were similar to the control. Dietary administration of 17 beta-estradiol at concentrations of 2.5 (both generations) and 10 and 50 ppm (P1 generation only) produced marked effects on the estrous cycle: decreased number of cycles, increased mean cycle length, and decreased number of normally cycling rats. The estrous cyclicity of rats fed 2.5 ppm 17 beta-estradiol appeared more severely affected in rats of the F1 generation than in rats of the P1 generation. Whether this increase in severity is related to an in utero exposure and/or greater mean daily intake of 17 beta-estradiol in the F1 generation rats in the postnatal period is unclear. Another goal of this study was to evaluate whether a single time point sampling strategy using cycling female rats could be used to detect compound-related changes in serum hormone concentrations. In evaluating a sampling strategy for measuring serum hormone levels, it appears that detection of compound-related alterations in serum hormone concentrations can be best detected by sampling during diestrus. Since the stage of the cycle dramatically influences hormone concentrations, large sample sizes (n = 50) are needed if serum hormone measurements are not matched with the stage of the cycle. The data indicate that this strategy of measuring serum hormone concentrations has utility in detecting compound-related effects within the confines of a traditional guideline study (subchronic, chronic, or multigenerational reproduction study).