Precocious germination was suppressed in cultured barley embryos by reduced O2, higher temperatures, elevated sucrose concentrations, and light. These parameters appear interrelated in that change in one may be offset partly by changes in others. Red light suppressed precocious germination, while far-red and blue did not. Seasonal differences in precocious germination responses were noted. Embryos excised from greenhouse plants in January tended to be dormant, those from plants grown in "warm-day" growth chambers, or those excised from field-grown plants in July, did not. It is thought that precocious germination in barley reflects the absence of dormancy factors which may be present in some embryos at an early stage in their development, and that dormancy in this sense does not imply quiescence since embryogenesis may continue.