Fibronectin (FN) is an important constituent of the extracellular matrix in bone. Its hormonal regulation in this tissue has not been extensively studied. We, therefore, examined the effects of PTH, estrogen, and transforming growth factor beta on the production of FN by human and rat osteoblast-like cells. Confluent cells were stabilized for 48 h under estrogen-replete (10(-9) M 17 beta-estradiol) conditions and then continued under these same conditions or withdrawn from estrogen for varying periods of time. PTH over the range 10(-11)-(10(-8) M caused a dose-dependent increase in FN production [P < 0.001 by analysis of variance (ANOVA)] such that at the highest dose, FN production was increased 11-fold. Estrogen withdrawal for 96 h caused a significant diminution in PTH-induced FN production (P < 0.005 by two-way ANOVA). Estrogen withdrawal over the of period 48-144 h caused a progressive diminution in PTH-induced FN production such that differences in mean values for estrogen-replete vs. deficient conditions were greater at 144 than 48 h (P < 0.05). The estrogen effect was titratable over the range 10(-11)-10(-9) M, and the inactive congener 17 alpha-estradiol failed to prevent the inhibitory effect of estrogen withdrawal on PTH-induced FN production. Interestingly, estrogen withdrawal had absolutely no effect on transforming growth factor-beta-induced FN production. Northern analysis demonstrated no effect of PTH on steady-state FN messenger RNA levels in Saos-2 cells under either estrogen-replete or estrogen-deficient conditions, suggesting that PTH effects an increase in FN production via a posttranscriptional mechanism in these cells. We conclude that PTH stimulates FN production in human and rat osteoblast-like cells, and under estrogen-deficient conditions this effect is significantly diminished. The modulatory effect of estrogen is not a universal phenomenon because transforming growth factor-beta-induced FN production is unaffected by estrogen withdrawal.