As frustration plays so important a rôle in modern psychological theory, the author decided to construct a standardized experimental frustrating situation and observe the reactions of patients to it. The subject is given a simple problem with only two possible solutions; after he has discovered these he is informed that there is a third solution. The situation thus created is regarded as one calculated to produce a feeling of frustration in the average person. The patient's behaviour is noted and various introspective data are inquired into. The behaviour of psychotic and psychoneurotic individuals is much more variable and unpredictable than that of normals. Schizophrenics deviate most from the normal, manic-depressives less, and paranoids still less. Obsessionals deviate more than other neurotics. The degree of deviation thus corresponds to the depth of libidinal regression.