A fitting for shutting off and/or regulating flows of substance for use in pressure ranges above 50 bar has a housing body and a valve disc which can be operated from a drive lying outside the housing body via a valve shaft. The housing body is provided with a maintenance opening that can be shut off in a pressure-tight manner by a self-sealing lid that has a seal. In order to allow objects that are as large as possible, in particular the valve disc, to be guided through the maintenance opening during inspection works, without increasing the weight of the housing body, the maintenance opening has a rectangular passage cross section. In contrast to known circular maintenance openings which are known in this pressure range, a rectangular passage cross section permits a smaller surface area for the passage cross section and therefore a lighter housing body.