Estimations of atmospheric grass pollen concentrations at an urban site were made at two hourly intervals on 67 days over a 5-year period when the average daily grass pollen concentration was 100/m3 or higher. Daily grass pollen concentrations rose from a morning minimum to reach a maximum in the early evening, falling gradually during the night. These results were compared with similar estimations made during 1 year at a rural site. Grass pollen concentrations at the rural site reached their maxima approximately four hours earlier than those at the urban site and corresponded well with maximum recorded values of sunshine, wind and temperature, and minimum values of relative humidity.