In vitro experiments were performed to examine the contractile responsiveness of guinea pig gallbladder smooth muscle as a function of developmental age. Gallbladder muscle strips from preterm (day 50-55 gestation), newborn (days 1-3 post partum), and 1-month-old animals were stimulated with agonists that initiate the contractile process either by activation of membrane receptors (acetylcholine and the octapeptide of cholecystokinin) or by membrane depolarization (potassium). Dose-response curves were constructed for each agonist in each age group and analyzed with respect to the maximal force developed and the pD2 value (negative logarithm of the dose of agonist which produces a one-half maximal response). The results can be summarized as follows: 1) when normalized for tissue cross-sectional area, the magnitude of the contractile response to each agonist increased with increasing developmental age; 2) the dose of agonist required to elicit a one-half maximal response was independent of developmental age. The data indicate that cholinergic and cholecystokinin receptors are present and functional on gallbladder smooth muscle prior to birth and that the force generating capacity of the tissue continues to develop after birth. A reduced contractility of the gallbladder in preterm and newborn animals as compared to young adults may partially explain the decreased choledochol bile flow seen in the neonate.