The ruler of Alba at the beginning of 900 was King Donald II (Donald the Madman), who had ruled since 889. This year, King Constantine II (Constantine the Middle-Aged) would take over.
The ruler of Gwynedd was King Anarawd ap Rhodri, who had ruled since 878 and would continue to do so until 916.
The ruler of Northumbria was King Eadwulf II (Eadwulf of Bamburgh), who had ruled since 895 and would continue to do so until 913.
The ruler of Strathclyde at this time was likely to have been King Dyfnwal.
The ruler of Wessex was King Eadweard I (Edward the Elder), who had ruled since 899 and would continue to do so until 924.