1. Fifteen primiparae, with no previous history of renal disease, were studied by intravenous pyelography; firstly when under three months pregnant, secondly within ten days of delivery and thirdly three months after the second pyelogram.2. Only five subjects showed a recognisable change. Two of these were followed up with a fourth pyelogram. The results are tabulated.3. It was concluded:(i) Changes in the calyces and pelves are not common in primiparae during pregnancy: such changes as do occur do not persist to a degree likely to give rise to chronic renal infection after delivery.(ii) The findings of dilatation of the renal pelves and calyces especially on the right side during pregnancy are not themselves pathological.(iii) The persistence of these changes is not necessarily pathological unless associated with both the clinical history and findings of chronic pyelitis.(iv) No useful information can be obtained from the study of the ureters in these cases by intravenous pyelography.4. It is considered that pyelographic abnormalities are likely to be due to a preceding pregnancy when:(i) A history of renal symptoms and signs following a normal pregnancy and delivery, is obtained.(ii) An imbalance in the pyelographic findings on the two sides, the right being more severely affected than the left.(iii) No other findings to explain the discovered changes can be demonstrated.