A PERTH veterinarian accused of the manslaughter of a teenager gave the boy oral sex and drugs, an inquest has been told.

David Michael Spry, 17, was found dead at the property of vet Marcel Christiaan-Rauch, east of Perth, in February 1999 after overdosing on ketamine, an animal tranquilliser.

A 2005 inquest led to Mr Christiaan-Rauch being charged with Mr Spry's manslaughter.

The vet, also charged with drug and sex offences against eight other men and boys, died this year before going to trial.

The inquest into David's death resumed this week.

A witness, whose name has been suppressed, told the Perth Coroner's Court today she was "disgusted" when she looked in Mr Christiaan-Rauch's bedroom window in 1996 and saw him performing oral sex on the youth.

"Marcel was kneeling in front of David giving him oral sex," she said.

"David was really out of it and looked like he was about to pass out."

She told the court boys would go into Mr Christiaan-Rauch's bedroom and inject themselves with ketamine.

She overheard these boys saying they were using four times the amount of ketamine necessary for their body weight.

David's father Ian Spry told the inquest today he did not suspect anything untoward when his son moved in with Mr Christiaan-Rauch in 1997 after dropping out of high school.

"I never had any reason to suspect anything was going on," Mr Spry said.

Mr Spry said David had grown close to Mr Christiaan-Rauch, who had taken him to Singapore on several occasions.

Another witness told the court Mr Christiaan-Rauch had fobbed off questions about David's health two days before the teenager died.

She has seen David stumbling, hitting cars and looked spaced out when she and her mother visited the vet surgery.

"Mum immediately asked Marcel 'What is the matter with David?'," she said.

"I got the feeling Marcel wanted to fob off the questions.

"When we got to the car Mum said to me 'I wouldn't be surprised if he (David) was dead by the morning'."

The inquiry continues tomorrow.