When cattle were superovulated with an FSH preparation containing no detectable LH (FSH-W), more viable embryos were recovered as compared with a standard preparation containing LH (FSH-P), with no change in the total number of ova + embryos recovered (Donaldson et al., 1986). To determine the basis for the increased embryo viability, we compared numbers of developing follicles and concentrations of estradiol in their follicular fluid at two times during the course of superovulatory treatment with FSH-P vs. FSH-W. Holstein heifers (n = 10/group) were injected with 3.5 mg of FSH-P or FSH-W twice daily beginning on Day 9 of the estrous cycle. Animals were ovariectomized either 48 h (Group 1) or 72 h (Group 2) after the initiation of treatment; heifers in Group 2 were also given a luteolytic injection of prostaglandin F2 alpha 24 h before ovariectomy. All follicles greater than S mm in diameter were dissected from the ovaries and follicular fluid was aspirated and stored frozen. Heifers injected with FSH-W had more follicles greater than 5 mm than heifers treated with FSH-P (21 + 4 vs. 11 + 3 in Group 1 and 28 + 5 vs. 20 +/- 5 in Group 2, respectively; p less than 0.05) and a significantly greater percentage of follicles from FSH-W animals were healthy (estrogen-active; 99 vs. 85% in Group 1 and 98 vs. 89% in Group 2, respectively; p less than 0.025). Estradiol concentrations in follicular fluid were more strongly correlated (p less than 0.001) with follicular size when heifers were treated with FSH-P vs. FSH-W.(ABSTRACT TRUNCATED AT 250 WORDS)