Characteristics such as brine and gas porosity and crystallographic features can have a profound impact on the mechanical properties of first-year sea ice. These characteristics vary spatially and temporally, and it is important in the development of constitutive models to address their variability in physically meaningful terms. A program of laboratory experiments on field cores of first-year sea ice has been conducted to aid in the development of such models. A thorough assessment of the bulk physical properties and microstructural characteristics of the ice has been carried out in conjunction with a detailed set of cyclic loading and creep experiments. Methodology was developed to calculate an orientation factor that determines the average shear stress resolved on the basal planes, given the background normal stress. Examination of the constitutive behavior using laboratory cyclic loading and constant load creep experiments revealed that the elastic, anelastic (time-dependent recoverable), and viscous strains varied systematically with the orientation factor. The observations also indicate significant brine porosity effects on the elastic, anelastic, and viscous components of strain. A recently developed constitutive model was expanded to include a frequency- and orientation-dependent viscous straining term, and the model predictions agreed well with the experimental observation.