SummaryIt is suggested that drug formulations should be developed with some reference to the diurnal variations in the symptoms of the disease for which they are prescribed. The suggestion is examined with regard to hay fever. As a result, interesting epidemiological data for the United Kingdom has been obtained. Hay fever is complained of more commonly in social classes A, B and C and in those living in London or South-East England. It is less common in the elderly. The principle symptom is sneezing, followed by stuffy nose, red itchy eyes, wheezing and coughing, in that order. These symptoms are commoner in the forenoon and are also, in general, more severe during this period. The theoretical implications of the data are discussed in the context of drug formulations.