The effect of harmaline has been tested on frog auricle by recording the mechanical tension of the muscle under voltage-clamp and processing the tisaue for electron microscopy. On a frog heart trabecula, the recordings show that harmaline induces a permanent and reversible contracture. The ultrastructural modifications were observed after ten min of incubation in harmaline-containing (3.5 mM) Ringer. Harmaline induces a swelling of the cells, with the formation of blebs, interpreted as resulting from a massive entry of water and ions (Gas+). A cytochemical lead technique was also used to follow the distribution of calcium; harmaline increases markedly the lead precipitate on the myofilaments as well as on the mitochondria and sarcoplasmic reticulum. The possible mechanisms of action of harmaline are discussed. KEY WORDS: Harmaline; Heart; Contracture; Calcium; Ultrastructure; Cytochemistry.