Posthepatic insulin availability has been evaluated by steady-state insulin turnover studies with 131I-insulin. Spontaneously delivered term (age 3.3 +/- 0.8 days) (mean +/- S.E.) and prematurely delivered lambs (betamathasone treated at 132 days) (age, 1.1 +/- 0.2 days) were compared with 4- to 5-month-old sheep. After a 7-hr fast, animals received 0.45% saline or 5.7 mg/kg/min glucose (0.06 ml/kg/min) for 6-hr followed by the tracer insulin infusion for 110 min. Plasma glucose, insulin, and immunoprecipitable insulin were measured sequentially during the steady state. Endogenous posthepatic insulin secretion and metabolic clearance rates were derived. Neither endogenous posthepatic insulin secretion rates nor metabolic clearance rates were different among the three groups of animals when either 0.45% saline or 5.7 mg/kg/min exogenous glucose infusions were compared. Secretory response of the pancreatic beta cell to continuous glucose infusion seems similar for the term and preterm lamb when compared to adult sheep.