1. The effects of amino acids on cytosolic free calcium concentration ([Ca2+]i) were measured, using fura-2 fluorescence imaging, in mouse pancreatic islets of Langerhans. 2. Slow [Ca2+]i oscillations appeared when isolated islets were incubated with a solution containing a mixture of amino acids and glucose at concentrations found in the plasma of fed animals. 3. In the presence of 11 mM glucose, alanine (5 mM) and arginine (10 mM) induced a transient rise in [Ca2+]i followed by an oscillatory pattern, while leucine (3 mM) and isoleucine (10 mM) triggered the appearance of slow [Ca2+]i oscillations. 4. Also in the presence of glucose (11 mM), tolbutamide (10 microM) increased the duration of the glucose-induced [Ca2+]i oscillations. While tolbutamide (10 microM) did not modify the leucine-induced slow oscillatory pattern, addition of diazoxide (10 microM) resulted in the gradual appearance of [Ca2+]i oscillations which resembled the glucose-induced fast oscillations. 5. Like stimulatory glucose concentrations (11 mM), glyceraldehyde (10 mM) induced fast oscillations of [Ca2+]i. 6. Fluoroacetate (2 mM) transformed leucine-induced slow [Ca2]i oscillations into fast [Ca2+]i oscillations. Iodoacetate (1 mM) completely inhibited any oscillatory pattern. 7. It is suggested that mitochondrially generated signals, derived from amino acid oxidative metabolism, acting in conjunction with glucose-signalled messengers, are very effective at closing ATP-dependent K+ channels (KATP+). 8. We propose that metabolic regulation of KATP+ channels is one of the mechanisms underlying the modulation of the oscillatory [Ca2+]i response to nutrient secretagogues.