I also think you shouldn't tell. People can have such prejudices against mental illness, and even if they're enlightened, it still might not stop them wondering about your efficiency there. If you do need to tell them, it might be better to wait until you've got to know them better before you start telling anybody, though if people are beginning to question you, I think Mellors gives a good suggestion about you being able to say that you have difficulties in the New Year period. If you think they're considering firing you though, then perhaps you should tell them, because perhaps they might give you some leeway, or just fire you earlier