The study, which is preliminary and has not yet been peer-reviewed, was designed to follow dosing and treatment procedures used for hospitalised Covid-19 patients in a large human trial.
It involved two groups of six rhesus macaques that were deliberately infected with SARS-CoV-2.
One group received the drug, which was developed by Gilead Sciences, and the other group did not.
The group receiving the drug got their first intravenous dose 12 hours after infection then every day subsequently for six days.
The scientists timed the initial treatment to occur shortly before the virus reached its highest levels in the animals’ lungs.
The treated animals showed significant improvement 12 hours after their first dose, a trend that continued over the week-long study.
One of the six treated animals showed mild breathing difficulty, while all six of the untreated monkeys had rapid and difficult breathing.
The amount of virus found in the lungs was significantly lower in the treated group compared to the untreated group. The treated group also had less lung damage.
Remdesivir was among the first drugs mooted as a treatment for the novel coronavirus and its randomised clinical trials are at an advanced stage.
The health news website Stat reported the drug had shown great efficacy at a Chicago hospital where patients who are part of those trials are being treated.
Remdesivir causes the virus to add mutations that can destroy it.
Meanwhile, Gilead Sciences Inc on Friday increased enrolment target by 3,600 for a trial testing remdesivir, a day after the report said the drug was showing promise.
Shares of the company rose eight per cent after Stat detailed rapid recovery in fever and respiratory symptoms in Covid-19 patients at the University of Chicago Medicine hospital.
The enrolment estimate in the trial run by Gilead was increased to 6,000 patients from 2,400 previously, according to clinicaltrials.gov, a register of clinical trials.
Brokerage Piper Sandler said rationale for the increase was to widen access to remdesivir and to gather additional data on the drug, noting that there were no additional changes to the goals.
Gilead did not immediately respond to a request for comment on increased enrolment.
The company has previously said it was shifting from a system of individual compassionate-use requests to expanded access programmes.
There are currently no approved treatments or vaccines for the novel coronavirus and remdesivir is one of the treatments that has captured investor attention.