F1 hybrids were bred from female GHS rats and male Wistar-Kyoto normocalciurics on: March 1997 (date 1), November 1998 (date 2) and January 2001 (date 3). Urinary calcium was measured at eight weeks of age in individual metabolic cages on days 4–10 on 13 g/d diet (0.6% calcium, 0.65% phosphorus, 0.24% magnesium, 0.40% sodium and 0.43% potassium, and 2.2 I.U. vitamin D3/gram diet) and averages and accumulated totals calculated using photometry with arsenazo III reaction. Genomic DNA was prepared using Puregene DNA Isolation Kits (Gentra Laboratories, Inc. Minneapolis, MN) and microsatellites amplified via PCR. Two hundred fifty-five were polymorphic (not all included here); 208 were amplied and scored for a total joint male-female map length of 1.41 cM with 176 markers and an average inter-marker distance of 8.0 cM in JoinMap 3.0. Linkage among markers was declared significant at an LOD of 3.0 or above.