In this project, we use diffusion tensor imaging (DTI) and quantitative relaxometry (QR) metrics to assess longitudinal changes in white matter (WM) throughout the postnatal rhesus macaque brain across the first year of life. DTI and QR metrics are measured at 3, 7, 13, 25, and 53 weeks of age in each of 35 non-human primates (NHPs) in this sample, for a total 175 datapoints (i.e., scans). Leveraging a previously published NHP WM atlas (Adluru et al., 2012; Zakszewski et al., 2014), we assess WM microstructure in 76 WM regions across the brain. DTI metrics include fractional anisotropy (FA), mean diffusivity (MD), radial diffusivity (RD), and axial diffusivity (AD); the QR metric assessed is the longitudinal relaxation rate (qR₁). These longitudinal data enable the direct comparison of DTI and QR metrics in very early life on a within-subject level, as well as robust, multimodal modeling of WM growth in infancy, providing insights into early postnatal neurodevelopment that may have important implications neuropsychiatric and neurodevelopmental disorders in childhood.