Physiological factors involved in seed dormancy in Cucumis sativus var. hardwickii (Royle) Alef. were investigated. Hardwickii seed (PI 183967) demonstrated a dormancy which required 100 days of after-ripening under ambient temperature (25°C) and humidity (approx. 60%) for 50% germination (T50). Removal of the seed coat released hardwickii embryos from dormancy and permitted 100% germination. Increasing the relative humidity above 60% in the atmosphere surrounding seeds stored at 25°C decreased the after-ripening time for T50 to between 42 and 56 days. After-ripening at 37 or 47°C reduced the T50 to 75 days, in comparison to a T50 of more than 100 days for temperatures below 37°C. When germinated on solutions of increasing osmolarities of polyethyleneglycol 4000, isolated embryos given longest after-ripening treatments had the greatest growth potential. Solvent extracts of dormant hardwickii seed were evaluated for inhibitors of germination in cucumber and curly cress bioassays. Seed dormancy in hardwickii is most likely not attributable to inhibitors present within hardwickii seed. It is hypothesized that after-ripening increases the growth potential of the embryo by allowing radicle penetration of the seed coat which may present a significant physical barrier in dormant seeds.