Attachment of the sheep metallothionein la promoter to the sheep growth hormone gene (oMT1a-oGH) has resulted in a closely regulated transgene in mice that can respond to elevated levels of zinc ions provided in the drinking water. The objective of this study was to explore the effects of zinc ion concentration on postweaning growth, fat content, organ size, and female reproduction. At weaning (3 weeks of age), hemizygous oMT1a-oGH mice were assigned randomly to receive 0, 12.5, 25, 37.5, or 50 mM ZnSO4 in distilled drinking water for 5 weeks. Males were killed at 8 weeks and selected organs and fat pads were weighed. Females received tap water from 8 to 10 weeks and then were mated to unrelated non-transgenic fertile males to obtain ovulation rate and embryo survival. Mortality rate was increased (P < 0.05) at 37.5 mM (15.2%) and 50 mM (33.3%) ZnSO4 compared with lower levels. Postweaning growth was highest at 12.5 mM ZnSO4 followed by 25 and 37.5 mM, respectively. At 50 mM ZnSO4, 8 week-old males were 20% smaller than males not receiving a supplement while females did not differ from females not receiving a supplement. Epididymal (EFP) and subcutaneous (SFP) fat pad weights of males decreased linearly (P < 0.01) with increased dosage of ZnSO4, while on a percentage of body weight basis, EFP and SFP decreased quadratically (P < 0.01). Males receiving 12.5 mM ZnSO4 had their EFP and SFP as a percentage of body weight reduced (P< 0.01) to 54.1% and 65.9% of controls (0 mM), respectively. As a percentage of body weight, liver spleen and kidney weights increased with doses of 12.5, 25 and 37.5 mM ZnSO4 compared with no supplemental zinc, while testis weight as a percentage of body weight was reduced (P < 0.05); percentage heart weight was only reduced at 12.5 mM (P < 0.05). No significant differences in female fertility were associated with ZnSO4 treatment. Females receiving ZnSO4 supplement had higher ovulation rates than those that received no supplement, but there were no differences (P > 0.05) in ovulation rates among the zinc levels. A decrease (P < 0.05) in pre- and post-implantation embryo survival was detected at 37.5 mM ZnSO4. These results indicate that zinc dosage affects phenotypic expression of the oMT1a-regulated oGH transgenic mice, and the dose responses are trait-dependent.