Just five minutes of exercise in a "green space" such as a park can boost mental health, researchers claim.
There is growing evidence that combining activities such as walking or cycling with nature boosts well-being.
In the latest analysis, UK researchers looked at evidence from 1,250 people in 10 studies and found fast improvements in mood and self-esteem.
The study in the Environmental Science and Technology journal suggested the strongest impact was on young people.
The research looked at many different outdoor activities including walking, gardening, cycling, fishing, boating, horse-riding and farming in locations such as a park, garden or nature trail.
The biggest effect was seen within just five minutes.
With longer periods of time exercising in a green environment, the positive effects were clearly apparent but were of a smaller magnitude, the study found.
Looking at men and women of different ages, the researchers found the health changes - physical and mental - were particularly strong in the young and the mentally-ill.
Green and blue
A bigger effect was seen with exercise in an area that also contained water - such as a lake or river.
Study leader Jules Pretty, a researcher at the University of Essex, said those who were generally inactive, or stressed, or with mental illness would probably benefit the most from "green exercise".