America's next national head count isn't until 1990, but the Census Bureau's dress rehearsal already is underway.

The census tests are being conducted in St. Louis, and in east central Missouri and eastern Washington state.Forms are being mailed to most residents of these areas, as was done in 1970 and 1980, although census takers will visit households in some sparsely settled areas. The effort is expected to continue into the summer.

The special counts are designed to be as much as possible like the effort planned for 1990, in order to get an understanding of what problems will develop before the work is begun nationwide, Census Director John G. Keane said.