Nitric oxide (NO) relaxes vascular smooth muscle in part through an accumulation of cGMP in the target cells. We hypothesized that a similar effect may also exist on collagen gel contraction mediated by human fetal lung (HFL1) fibroblasts, a model of wound contraction. To evaluate this, HFL1 cells were cultured in three-dimensional type I collagen gels and floated in serum-free DMEM with and without various NO donors. Gel size was measured with an image analyzer. Sodium nitroprusside (SNP, 100 µM) significantly augmented collagen gel contraction by HFL1 cells (78.5 ± 0.8 vs. 58.3 ± 2.1, P < 0.01), whereas S-nitroso-N-acetylpenicillamine, 5-amino-3-(4-morpholinyl)-1,2,3-oxadiazolium chloride, NONOate, and N^G-monomethyl-L-arginine did not affect the contraction. Sodium ferricyanide, sodium nitrate, or sodium nitrite was not active. The augmentory effect of SNP could not be blocked by 1H-[1,2,4]-oxadiazolo-[4,3-a]-quinoxalin-1-one, whereas it was partially reversed by 8-(4-chlorophenylthio) (CPT)-cGMP. To further explore the mechanisms by which SNP acted, fibronectin and PGE₂ production were measured by immunoassay after 2 days of gel contraction. SNP inhibited PGE₂ production and increased fibronectin production by HFL1 cells in a concentration-dependent manner. CPT-cGMP had opposite effects on fibronectin and PGE₂ production. Addition of exogenous PGE₂ blocked SNP-augmented contraction and fibronectin production by HFL1 cells. Therefore, SNP was able to augment human lung fibroblast-mediated collagen gel contraction, an effect that appears to be independent of NO production and not mediated through cGMP. Decreased PGE₂ production and augmented fibronectin production may have a role in this effect. These data suggest that human lung fibroblasts in three-dimensional type I collagen gels respond distinctly to SNP by mechanisms unrelated to the NO-cGMP pathway.