Glycogen synthesis from D-[1- 1 3 C]glucose was observed in the perfused rat heart by 1 3 C-NMR spectroscopy at 62.9 MHz. The glycogenogenesis was stimulated by pretreatment of the animals with isoprenaline. Whereas in hearts from control rats the incorporation of D-[1- 1 3 C]glucose into the glycogen remained below the detection threshold, 5 min proton-decoupled 1 3 C-NMR spectra revealed, in hearts from treated rats, a significant labelling of the glycogen within the first minutes of the perfusion and a further linear increase of the glycogen resonance for up to 25 min. This model was used to monitor the appearance of 1 3 C-labelled lactate during ischemia.