The monthly threshold for paying NI is �420, therefore if you have exceeded this in your 2nd job you will pay contributions. This threshold applies to each job you have - however if you are already earning more than around �33500 in your main job, you will already be paying the maximum at 11%, and you may be able to defer payment of contributions on your second job until the end of the tax year. You will still be liable to 1% contributions on the second job, and you will be sent a bill for them. In strictness you should apply before the tax year starts, but in practice applications are accepted until around february in the tax year concerened.