The effects of triiodothyronine (T<sub>3</sub>) on the force frequency responses of isometrically contracting rabbit papillary muscles were studied in the presence of d-glucose, pyruvate or butyrate. The stimulation frequency was varied from 0.1 to 0.5 Hz, and the maximum developed tension and its maximum (T<sub>mx</sub>) and minimum (T<sub>mn</sub>) time derivative were measured. T<sub>3</sub> concentrations ranged from 0.1 to 4.0 ng/ml. The addition of T<sub>3</sub> resulted in a substrate-dependent increase in twitch tension; with the largest increases being: d-glucose 118 ± 7%, pyruvate 143 ± 6%, butyrate 123 ± 11 %; T<sub>mx</sub>: d-glucose 121 ± 8%, pyruvate 157 ± 5%, butyrate 138 ± 12%, and T<sub>mn</sub>: d-glucose 150 ± 10%, pyruvate 159 ± 6%, butyrate 163 ± 14%. All three measures of contractility showed a dose-dependent increase reaching a maximum value at a T<sub>3</sub> concentration between 1 and 2 ng/ml. These data show that T<sub>3</sub> induces an inotropic response in rabbit papillary muscles which is manifested within, ∼30 min, and that the greatest increa
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