IT is commonly believed that the advent of rain in regions long subject to drought or desert conditions is followed by an abnormally rapid response in natural seed germination in the areas concerned. As such climatic conditions are universally favourable to the accumulation and liberation of petrichor1 from clays and other silicate minerals, as components of soils, it seemed possible that some of the substances which accompany the argillaceous odour of petrichor might favourably influence the germination of seeds. Our experiments have shown that there is a marked effect but that it is unexpectedly characterized by a retardation rather than an acceleration of germination and early plant growth. It was observed that minute amounts of the oily extracts prepared from various mineral sources significantly delayed the germination and growth of cress, mustard and mixed grass seeds. Further, it was found that seeds, shown on moistened kaolinized granite or other source material which had been previously exposed to the atmosphere under the warm, dry conditions necessary for the development of petrichor, took longer to germinate and had a slower growth rate than seeds sown on source material that had not been exposed or that had been steam-distilled and oven-dried before use. An exposure period of 2-3 weeks at 60°-75° F with a relative humidity of less than 70 per cent was sufficient to produce a small difference in growth rate. This inhibitory effect increased with prolonged exposure of the source material.