1. Cardiac fibroblasts play an important role in the pathophysiology of cardiac remodelling induced by hypertension and myocardial infarction by undergoing proliferation and depositing extracellular matrix proteins such as collagen. We have examined the effects of atrial natriuretic peptide (ANP) on proliferation and collagen synthesis by adult rat and human cardiac fibroblasts in culture. 2. In cells from both species radioligand studies using 125I-ANP suggested that the majority of binding sites (> 85%) were non-guanylyl cyclase-linked (NPR-C subtype). Nonetheless ANP (10(-9) to 10(-6) M), in the presence of zaprinast, an inhibitor of phosphodiesterase 5 (PDE5), increased fibroblast cyclic GMP levels 3-5 fold in a concentration-dependent manner (P < 0.05). 3. ANP (10(-11) to 10(-6) M), a NPR-C ligand, C-ANF4-23 (10(-11) to 10(-6) M) and zaprinast alone had no significant effect on either basal or serum-stimulated DNA synthesis or fibroblast number. In combination with zaprinast (10(-5) M), however, ANP (10(-9) to 10(-6) M) but not C-ANF4-23 (10(-7) M) inhibited markedly both basal and stimulated fibroblast mitogenesis, an effect reproduced by 8-bromo-cyclic GMP (10(-5) to 10(-3) M). 4. Collagen synthesis, determined by measuring hydroxyproline levels, was stimulated with transforming growth factor-beta1 (40 pM), angiotensin II (10(-7) M) or 2% foetal bovine serum. The increase in collagen production, normalised by cell number, was reduced dramatically (to at or near basal production) by ANP (10(-9) to 10(-7) M) but not C-ANF4-23 (10(-7) M) in the presence of zaprinast. Again 8-bromo-cyclic GMP (10(-5) to 10(-3) M) reproduced the effect. 5. ANP is capable of inhibiting collagen synthesis in adult rat and human cardiac fibroblasts via cyclic GMP, a property unmasked and enhanced by inhibition of PDE5.