In vitro zygotic and somatic embryogenesis procedures for wheat have been improved by simulating in ovulo nutritional, hormonal and dissolved oxygen (dO2) conditions. However, diurnal fluctuations in these conditions during early embryony are not well characterized. In this study, dO2 and water-soluble carbohydrate Levels in wheat kernels were determined after 8 h of light and 8 h of dark at approximately 6, 12 and 18 day post anthesis (DPA). Clark style O2 microelectrodes, having a tip diameter of approximately 115 microm, were inserted into intact kernels immediately distil to the developing embryo, and dO2 levels were recorded at 50 microm intervals into the center of kernels. High-performance anion exchange chromatography with pulsed amperometric detection was used to quantify carbohydrate levels in endosperm sap. dO2 levels in the chlorophyllous layer of the pericarp reached 190 mmoLm(-3) during the day, which probably represents, because of photosynthesis, a supersaturated O2 condition relative to the external environment (21% O2). At the embryo surface, dO2 levels at 6 DPA ranged from 135 to 170 mmolm(-3). At 12 and 18 DPA, dO2 levels at the embryo axis ranged from 100 to 150mmolm(-3). At all three stages, dO2 levels in the center of the endosperm were below 13 mmolm(-3). Extreme fluctuations in carbohydrate levels were observed diurnally during rapid seed fill (12DPA). Levels of sucrose and short-chain fructans were much higher during the day than during the night. In contrast, fructose, glucose, and myo-inositol levels were much higher during the night than during the day. By 18DPA (hard dough stage), carbohydrate levels tended to be similar during the day and night. These dynamic fluctuations may assist in regulating embryony in ovulo, and their simulation might improve the development of somatic and zygotic embryos in vitro.