In recent years, extensive efforts have been made in developing near-infrared optical techniques to be used in detection and diagnosis of breast cancer. Variations in optical properties of normal breast tissue set limits to the performance of such techniques and must therefore be thoroughly examined. In this paper, we present intra- and intersubject as well as contralateral variations of optical and physiological properties in breast tissue as measured by using four-wavelength time-resolved spectroscopy (at 660, 786, 916 and 974 nm). In total, 36 volunteers were examined at five regions at each breast. Optical properties (absorption, μ_a, and reduced scattering, μ'_s) are derived by employing diffusion theory. The use of four wavelengths enables determination of main tissue chromophores (haemoglobin, water and lipids) as well as haemoglobin oxygenation. Variations in all evaluated properties seen over the entire breast are approximately twice those for small-scale heterogeneity (millimetre scale). Intrasubject variations in optical properties are almost in all cases below 20% for μ'_s, and 40% for μ_a. Overall variations in water, lipid and haemoglobin concentrations are all in the order of 20%. Oxygenation is the least variable of the quantities evaluated, overall intrasubject variations being 6% on average. Extracted physiological properties confirm differences between pre- and post-menopausal breast tissue. Results do not indicate systematic differences between left and right breasts.