Two radioimmunoassays (RIA-A and RIA-B) were used to estimate morphine equivalents (ME) in rabbit serum after i.v. administration of morphine or morphine-3-glucuronide (M-3-G). RIA-A and RIA-B differed only with respect to the particular pool of antibodies used, and both were elicited from rabbits immunized with a 2-diazomorphine-protein conjugate. In vitro inhibition studies indicated the molar ratio of M-3-G/morphine which inhibited 3H-morphine-antimorphine binding by 50% (150) was 90 and 14 for RIA-A and RIA-B, respectively. RIA-B 150 values were dependent on the final protein concentration of the incubation mixture. RIA-A and RIA-B estimates of the ME per milliliter were in agreement in serum samples obtained from rabbits after i.v. morphine 0.01 mg/kg; however, after higher doses only samples obtained in the first minutes agreed and at later times RIA-B estimates exceeded RIA-A by increasing amounts. After i.v. M-3-G (1.0 mg/kg) RIA-B estimated 20 to 25 times more ME per milliliter than RIA-A. After rapid i.v. administration of morphine (1 or 5 mg/kg) the disappearance curves obtained with RIA-B were multiphasic and contained a prolonged terminal phase with an estimated half-life of 24 hours. The higher estimates obtained with RIA-B are best explained by cross-reactions with M-3-G or other morphine metabolites. With RIA-A the mean serum morphine clearance (86 ml/min/kg) and half-life of the elimination phase (74 minutes were independent of dosage. The least complicated pharmacokinetic model which describes the disposition of morphine in the rabbit is the open two-compartment model.