Contractile force and intraventricular conduction during progressive ouabain infusion were examined in 15 dogs using a Walton-Brodie strain gauge arch and sequential atrial and bundle of His pacing. This permitted: (1) contractile force determination; (2) overdrive of arrhythmias; (3) maintenance of normal contraction sequences; (4) 'normal' ventricular depolarization; (5) rate control; (6) conduction measurements of the H-V interval (His-Purkinje conduction), QRS (intramyocardial conduction), and H-S interval (total intraventricular conduction). Contractile force increased 21.2 plus or minus 4.3% at the onset of toxicity. After toxicity, there was a significant further increase (P smaller than 0.01) to 50.1 plus or minus 12.4%. However, immediately before ventricular fibrillation, a 43.8 plus or minus 8.2% decrease occurred (P smaller than 0.01). H-V time showed no change (from 30 plus or minus 2.7 to 31.5 plus or minus 2.4 ms) at the onset of toxicity but after toxicity, it lengthened to 40.5 plus or minus 3.1 ms (P smaller than 0.05). QRS did show significant prolongation (69.5% plus or minus 5.3 to 79.5 plus or minus 6.9 ms; P smaller than 0.05) at the onset of toxicity, but this was more marked (79.5 plus or minus 6.9 to 130.5 plus or minus 8.1 ms; Pplus or minus 0.01) after toxicity. H-S time was significantly prolonged (99.5 plus or minus 6.2 to 111.0 plus or minus 8.9 ms) before (P smaller than 0.02) and after (P smaller than 0.01) toxicity (111.0 plus or minus 8.9 to 171.1 plus or minus 10.6 ms). During toxicity there is progressive increase in contractile force with continued ouabain infusion Progressive prolongation of intramyocardial conduction occurs in nontoxic and toxic doses, but His-Purkinje conduction is prolonged only in supratoxic doses.