Sodium nitroprusside (SNP), a nonreceptor mediated stimulant of soluble guanylate cyclase, and atrial natriuretic factor, a receptor-dependent stimulator of particulate guanylate cyclase, mediate relaxation responses by increasing intracellular cGMP. Thisin vitrostudy was designed to compare the ontogeny of relaxation responses to SNP and atrial natriuretic factor in the guinea pig thoracic aorta. Aortic rings from fetuses at 55-60 d gestation (term=68 d), 1- to 3-d-old newborn, and 12-wk-old adult Hartley guinea pigs were mounted in an organ bath, bathed in Kreb's solution, and connected to a force-displacement transducer to measure isometric tension. Relaxation responses to SNP and atriopeptin III were studied with the vessels at optimal resting tension and after preconstriction with an EC85concentration of norepinephrine. SNP-mediated relaxation showed a significant increase in sensitivity with development among the three age groups (p<0.05). Methylene blue, an inhibitor of soluble guanylate cyclase, produced no inhibition of relaxation to SNP in fetal aortae, significantly decreased responses along the straight portion of the concentration-response curve in newborn aortae (p<0.05), and significantly shifted the concentration-response curve to the right (p<0.05) in adult aortae; but did not prevent vessels from relaxing almost 100% in any age group. However, atriopeptin III-mediated responses were similar in the three age groups and were unaffected by methylene blue. These results suggest that1) sensitivity to SNP increases with age from fetal through adult life;2) relaxation mediated by atriopeptin III is similar during development;3) methylene blue does not affect SNP mediated relaxation in fetuses but progressively decreases sensitivity to SNP in newborns and adults; and4) methylene blue does not affect atriopeptin III-mediated relaxation in any age group.