Mask ports 180 for attaching supplemental oxygen tubes 510 or measurement devices to a respiratory mask are downwardly directed and recessed into the base 110 of the mask frame 100. The ports may comprise a pair of downwardly extending tubular spigots 185 each housed in a respective recess 190 in the base 110, with a shallow bridging recess 290 therebetween for receiving a bridging piece 300 of a closure cap 280.