Squirrel monkeys, owls and camels are among the creatures lined up for the annual weigh-in at ZSL London Zoo.
Keepers are recording the height and weight of more than 20,000 animals to monitor their health and wellbeing.
The details will be added to the Zoological Information Management System, a database shared with zoos all over the world that helps zookeepers compare important information on thousands of endangered species.
Zookeepers employ various tactics to entice the animals to stand up and be measured.
Penguins are encouraged to walk over scales as they line up for their morning feed, while fun training activities help persuade squirrel monkeys to pop on to the scales.
The checks help keepers ensure the animals are healthy, eating well and growing at the rate they should.
Weight is a particularly important indicator of health and wellbeing, and can also help zookeepers detect and monitor pregnancies.