Diurnal variations of isoprene and monoterpenes in the atmosphere were precisely measured at an interval of 1--2 hours in the rainly season and in fine midsummer in an agricultural area in Japan. It was observed that the mixing ratio of isoprene was much larger in fine daytime than in nighttime or in cloudy daytime; in contrast, a higher mixing ratio of monoterpenes was usually observed in nighttime. In fine midsummer the concentration of atmospheric isoprene increased gradually after midnight, reaching its maximum late in the afternoon, and decreased rapidly after sunset. The significant difference between atmosphere isoprene and monoterpenes was explained by suggesting that isoprene emission is more strongly affected by temperature and/or solar radiation. ¿ American Geophysical Union 1988