The role of reduced and oxidized glutathione in plant development was investigated using wild carrot suspension cultures. Concentrations of GSH are lower in developing than in proliferating carrot cultures. Addition of 0.3 mM buthionine sulfoximine (a glutathione synthesis inhibitor) to developing cultures decreased the cellular GSH levels and enhanced somatic embryogenesis while addition of 0.6 mM GSH increased the cellular GSH levels and inhibited embryogenesis. Additions of GSH and buthionine sulfoximine to developing cultures also indicated that buthionine sulfoximine is acting specifically to lower GSH levels and not through some nonspecific toxic effect. These results provide evidence that the levels of GSH are important in determining whether carrot cells develop into somatic embryos or grow proliferatively.