Summary 1. There is a normal histidinuria both in men and women. 2. The determination of histidinuria by the colorimetric method based on bromuration of histidin is neither accurate nor specific. 3. The urinary excretion of histidin increases during pregnancy. In some cases this increase may be slight and then can only be proved by comparing histidin excretion during and without pregnancy in the same patient. 4. Hyperhistidinuria in pregnancy is a remarkably precocious phenomenon. It seems to be simultaneous with fecondation and it is already measurable a few days after it. 5. The measurement of histidinuria may reveal itself as a useful test for the diagnosis of pregnancy if a limit between the values of histidinuria during and without pregnancy can be established by statistical methods from a great number of cases. This technic might be able to avoid the uncertainties related to biological methods for the diagnosis of pregnancy.