Leydig cells, which produce the primary male steroid hormone testosterone (T), express the two estrogen receptor (ER) subtypes, ERα and ERβ, and have the capacity to convert testosterone to the natural estrogen 17β-estradiol. Thus, Leydig cells are subject to estrogen action. The development of transgenic mice that are homozygous for targeted deletion of genes encoding the ER subtypes provides an opportunity to examine the role of estrogen in Leydig cell function. In this study androgen biosynthesis was analyzed in Leydig cells from mice that were homozygous for targeted deletion of the ERα gene (αERKO). T production by αERKO Leydig cells was 2-fold higher than that in wild-type (WT) cells. Serum T levels were accordingly higher in αERKO compared with WT mice (5.1 ± 1.1 vs. 2.2 ± 0.4 ng/ml; P≤ 0.01) as were serum LH levels (1.31 ± 0.3 vs. 0.45 ±0.08 ng/ml; P≤ 0.01). Mice that were treated with the pure antiestrogen ICI 182,780 at 100 µg/kg·d for 7 d, effectively abrogating ER-mediated activity, also had 2-fold elevations in the serum levels of LH (1.15 ± 0.3 vs. 0.45 ±0.2 ng/ml) and T (4.3 ± 1.1 vs. 2.2 ± 0.2 ng/ml; P≤ 0.01). Increased androgen biosynthesis by αERKO Leydig cells was associated with higher steroidogenic enzyme activity, especially of cytochrome P450 17α-hydroxylase/17-20 lyase (P450_17α) and 17β-hydroxysteroid dehydrogenase (17β-HSD), as measured by conversion of radiolabeled steroid substrates to T or its precursors. The largest increases in enzymatic activity were observed for P450_17α (423 ± 45 pmol/min·10^6 cells in αERKO Leydig cells vs. 295 ± 27 pmol/min·10^6 cells in WT cells; P < 0.01). Consistent with steroidogenic enzyme activity, the testis of αERKO mice expressed higher steady state mRNA levels for steroidogenic acute regulatory protein and two enzymes involved in androgen biosynthesis, P450_17α and17β-HSD type III, as determined by semiquantitative RT-PCR. Compared with the controls, higher steady state mRNA levels for steroidogenic acute regulatory protein and P450_17α were also measured in the testis of ICI 182,780-treated mice. In a second set of experiments estrogen administration reduced serum LH and T levels in WT controls, whereas αERKO mice were unaffected. Although exposure of WT and αERKO Leydig cells to estrogen in vitro did not affect androgen biosynthesis, incubation with ICI 182,780 reduced T production by WT, but not αERKO, Leydig cells occasioned by incubation with ICI 182,780 suggests that of the ER subtypes, ERα has a regulatory role in Leydig cell steroidogenic function.