Leading Brisbane trainer Robert Heathcote is calling for a change to the rule which says he must use a claiming apprentice to replace junior rider Matt McGillivray who has been stood down over a positive drugs test.

Heathcote booked McGillivray on Wednesday for three of his runners on Saturday - Bewhatyouwannabe, Jopa and Dream of Slips.

On Thursday, stewards stood the apprentice down for an indefinite period after he returned a positive test to amphetamine.

The rules say a claiming apprentice must be replaced by another claiming apprentice unless there are none available.

Heathcote had wanted to replace McGillivray on Dream of Slips with senior rider Glen Colless but stewards ruled against the move.

"They are saying there are other claiming apprentices available and I have to use them. Why can't I change my mind?" Heathcote said.

"I booked Matt on Wednesday. I can see some claims for the rule to apply an hour before a race when many punters have already placed their bets."

"But this is Friday morning and I don't see why I can't change my mind and use a senior rider if he is available."

Heathcote said he would be taking up the matter of a rule change with stewards.

Meanwhile, stewards will open an inquiry into McGillivray's positive test as soon as possible.

"We hope to get it started on Monday or Tuesday. He is stood down until the inquiry is completed so it is in everyone's interest that it is completed quickly," chief steward Allan Reardon said.