A wood.
Three witches are seen against a background of thunder and lightning, intoning a chorus that culminates in an infernal dance («Le sorelle vagabonde / van per l’aria, van sull’onde »). Enter Macbeth and Banquo, generals of the Scottish army, to whom the weird sisters foretell a propitious future: Macbeth will soon be named thane of Cawdor by Duncan king of Scotland, and will later ascend to the throne. Banquo, though without ever reigning, will have royal descendants.The two men’s amazement reaches its pitch when the king’s messengers inform Macbeth that Duncan has created him thane of Cawdor.When the two generals have left the scene with their retinue, the witches chant the “stretta dell’introduzione”  («S’allontanarono!»).

A hall in Macbeth’s castle.
Lady Macbeth enters, reading a letter from her husband in which he tells her of the witches’ prophecies. The Andantino of her Cavatina expresses her deep yearning for power («Vieni! t’affretta! Accendere»). Having heard from a servant that her husband is on his way to the castle with king Duncan who is to be their guest for the night, Lady Macbeth gives vent to her criminal designs.

She has in fact taken it into her mind to murder Duncan so that her husband can put on the crown of Scotland (cabaletta: «Or tutti sorgete, ministri infernali»). Macbeth enters with the king’s retinue, looking hesitant. Only his wife’s determination gives him the strength to commit the crime. The scene culminates in a duet by the evil pair («Fatal mia donna! un murmure») during which Macbeth, seized with remorse, dares not take the dagger back to the scene of the crime. Lady Macbeth tears the bloodstained weapon from her husband’s hands and has no hesitation in returning it herself to the room in which the king was slain, so that the blame will fall upon the guards. When the murder of Duncan is discovered, the noble Macduff in horror informs the courtiers. The act closes amidst general dismay («Schiudi, inferno, la bocca ed inghiotti»).