The first detailed day to day survey of pollen deposition in any part of Great Britain was made in I942 (Hyde & Williams, I944). It was carried on for twelve months and was concerned with one station only, and the conclusions drawn therefrom could only be tentative. With the co-operation of the heads of the various institutions named in pp. 40I-403 a survey embracing eight stations and extending from south Wales and London north to Aberdeen was therefore carried out in I943. The aims of this survey were to determine what were the constituents of the pollen rain in Great Britain and the variations in its composition from day to day and from place to place in relation to probable sources in the respective neighbourhoods concerned and in relation to the weather conditions experienced. The survey was made as part of a programme of research on pollen as an allergic agent. It was hoped that it might also contribute toward an understanding of data obtained from the pollen analysis of peat. The same general procedure for the catching of pollen from the air was followed at all stations. A 3 X I in. glass microslide previously coated with glycerin jelly containing basic fuchsin was placed horizontally in an apparatus situated on the roof or in the grounds of the institution referred to. The apparatus (Fig. i) was designed to allow air to pass freely over the slide while at the same time protecting it from rain other than that experienced during violent squalls. It consisted essentially. of two horizontal circular disks of asbestos bolted together i i in. apart, together with a metal spring clip to hold the slide in position 4 in. below the upper disk. The slides, previously prepared, were sent by post to all stations outside Cardiff. One was placed every morning at io a.m. in the apparatus, left there for 24 hr. and replaced at the end of that time by another and so on throughout the year. After they had been exposed the slides were repacked and posted back to Llandough, where, immediately on receipt, they were mounted each with two 7 in. cover-slips. Later the various types of pollen present under the whole of one coverslip (5 sq.cm.) were identified and counted. The primary data so obtained were set out in a series of tables, I09 in all, which may be consulted on application to the writer.