A subsea wellhead assembly includes a wellhead body which disposed to support an isolation sleeve for a production tree and to support below the isolation sleeve a production casing hanger for a production casing string. The body also supports a second casing string outside the production casing string whereby to form an annulus between the casing strings. The body includes a passageway which is in communication with the annulus near a lower end of the body and extends upwardly within the wall of the body towards the upper end of the body. A valve is accommodated in the upper part of the body and provides controlled communication between the passageway and a port which can be coupled to the isolation sleeve. The valve is controllable from a production tree in particular is moveable between an open position and a closed position in response to fluid pressure applied to the valve from a production tree by way of the isolation sleeve.