After previously examining 12 compounds with known endocrine activities, we have now evaluated 4 additional compounds in a Tier I screening battery for detecting endocrine-active compounds (EACs): a weak estrogen receptor (ER) agonist (coumestrol; COUM), an androgen receptor (AR) agonist (testosterone; TEST), a progesterone receptor (PR) agonist (progesterone; PROG), and a PR antagonist (mifepristone; RU486). The Tier I battery incorporates 2 short-term in vivo tests (5-day ovariectomized female battery; 15-day intact male battery) and an in vitro yeast transactivation system (YTS). The Tier I battery is designed to identify compounds that have the potential to act as agonists or antagonists to the estrogen, androgen, progesterone, or dopamine receptors; steroid biosynthesis inhibitors (aromatase, 5alpha-reductase, and testosterone biosynthesis); or compounds that alter thyroid function. In addition to the Tier I battery, a 15-day dietary restriction experiment was performed using male rats to assess confounding due to treatment-related decreases in body weight. In the Tier I female battery, TEST administration increased uterine weight, uterine stromal cell proliferation, and altered hormonal concentrations (increased serum testosterone [T] and prolactin [PRL]; and decreased serum FSH and LH). In the male battery, TEST increased accessory sex gland weights, altered hormonal concentrations (increased serum T, dihydrotestosterone [DHT], estradiol [E2], and PRL; decreased serum FSH and LH), and produced microscopic changes of the testis (Leydig cell atrophy and spermatid retention). In the YTS, TEST activated gene transcription in the yeast containing the AR or PR. In the female battery, COUM administration increased uterine weight, uterine stromal cell proliferation, and uterine epithelial cell height, and increased serum PRL concentrations. In the male battery, COUM altered hormonal concentrations (decreased serum T, DHT, E2; increased serum PRL) and, in the YTS, COUM activated gene transcription in the yeast containing the ER. In the female battery, PROG administration increased uterine weight, uterine stromal cell proliferation, and uterine epithelial cell height and altered hormonal concentrations (increased serum progesterone and decreased serum FSH and LH). In the male battery, PROG decreased epididymis and accessory sex gland weights, altered hormonal concentrations (decreased serum T, PRL, FSH, and LH; increased serum progesterone and E2), and produced microscopic changes of the testis (Leydig cell atrophy). In the YTS, PROG activated gene transcription in the yeast containing the AR or PR. In the female battery, RU486 administration increased uterine weight and decreased uterine stromal cell proliferation. In the male battery, RU486 decreased epididymis and accessory sex gland weights and increased serum FSH and LH concentrations. In the YTS, RU486 activated gene transcription in the yeast containing the ER, AR, or PR. Dietary restriction data demonstrate that confounding due to decrements in body weight are not observed when body weight decrements are 10% or less in the Tier I male battery. In addition, minimal confounding is observed at body decrements of 15% (relative liver weight, T3, and T4). Hence, compounds can be evaluated in this Tier I at levels that produce a 10% decrease in body weight without confounding of the selected endpoints. Using the responses obtained for all the endpoints in the Tier I battery, a distinct "fingerprint" was produced for each type of endocrine activity against which compounds with unknown activity can be compared. These data demonstrate that the described Tier I battery is useful for identifying EACs and they extend the compounds evaluated to 16.