Ethical: morally correct; noble, worthy; virtuous;
It is ethical that monkeys are used in experiments, if they are not in any form of danger, because the monkeys will learn things that wild monkeys do not. They might learn how to communicate or how to operate a pinball machine. This allows them to gain a new life of understanding how things work, and how to work as a team. (This includes sharing)Team work and sharing is essential because if they want to move away a rock to do something, they have to work together in order to do what they want to do, like getting food. This is when sharing comes in play. If there is one banana, for an instance, they have to share so that they will both get some banana and they will also avoid a fight.
They can take all this new knowledge back to the wild if they got released. Then some could survie in the wild, if they are strong enough, but most monkeys cannot go back into the wild because they got used to hanging around with humans and they cannot really take care of themselves in the wild.
Over all, it is ethical that we use monkeys in harmless experiments because the monkeys, in the end, will learn something new.