1. 1. A single neonatal treatment with a cardioactive glycoside (ouabain, digoxin) altered the response of the adult rat to digitaloid treatment. 2. 2. As demonstrated by RIA, re-exposure to digoxin at 2 months of age was followed within 30 min by a more than twofold increase in serum digoxin over the not pretreated control and, although a steady concentration decrease followed, the experimental rats still had a higher serum digoxin level than the controls at 4 h. 3. 3. In the not presensitized control group the serum digoxin peak appeared at 60 min at a considerably lower level than the 30-min maximum of the experimental rats. 4. 4. Neonatal pretreatment with ouabain depressed myocardial ouabain binding, but enhanced the Na+ K+-ATPase activity. 5. 5. The above differences were conspicuous in the functional response, yet a greater atrial response to the positive inotropic action of K-stophantoside and a greater ventricular response to beta adrenergic excitation were readily seen in the experimental group. 6. 6. It follows that not only hormones, but also other ligands acting at receptor level can be regarded as potential inducers of imprinting.
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