Police are looking for two people who assaulted a 58-year-old motorist with a hammer in Perth's north-east.

The man was driving on Guildford Road in Maylands on Saturday night when an oncoming vehicle flashed its lights and sounded its horn.

When he pulled over on a nearby side street, a man and a woman got out of their car, struck him with a hammer and stole his wallet.

The man was later treated in hospital.

The male offender is described as fair-skinned with light coloured hair and the female offender is in her mid-twenties and possibly pregnant.

Meanwhile, the arson squad will investigate the cause of the fire that destroyed a gymnasium in Bibra Lake.

Authorities were called to the Solar Pass property just after late on Saturday night.

The fire caused $100,000 damage.