AbstractWhile the use of salivary cortisol as an index of hypothalamic–pituitary–adrenal (HPA) axis activation has increased rapidly in human studies, few non-human primate studies have used saliva samples. Nearly 300 h of behavioural data and over 400 saliva samples were collected from three young adult, male western lowland gorillas to document the feasibility and effectiveness of using salivary cortisol as an index of HPA activation in gorillas. Cortisol concentrations were significantly higher in morning samples than in afternoon samples, and there was a significant decrease in morning cortisol concentrations across the study period. Additionally, acute increases, followed by a return to baseline concentrations of cortisol were observed. Salivary cortisol concentration was found to correlate across individuals, indicating potential psychological attunement to environmental and/or social variables in these animals. Although no clear relationship between behaviour and cortisol concentrations was established, these results indicate that salivary cortisol is an effective technique for documenting HPA activity over an extended period of time, as it allows for detection of diurnal variation as well as acute changes in salivary cortisol concentrations.