The behavior of the activities of GMP synthetase (xanthosine-5'-phosphate: L-glutamine amino-ligase(AMP-forming),EC 6.3.5.2) and GMP kinase (ATP: (d)GMP phosphotransferase,EC 2.7.4.8) was elucidated in cytosol preparations of rat tissues, including fetal, neonatal and regenerating liver, in a transplantable kidney tumor, and in a spectrum of 11 hepatomas of different growth rates. GMP kinase activity was 60-fold or more higher than GMP synthetase activity in all of the examined tissues. GMP synthetase activity was increased in all hepatomas and in the kidney tumor, compared to control tissues, reaching 5.5-fold the normal liver values in the most rapidly growing hepatoma. This increase correlated with the tumor growth rates. GMP kinase activity showed no consistent pattern of alteration in the tumors. In both fetal and neonatal rat liver the activity of GMP synthetase was 2.5-times higher than in livers of adult rats, but GMP kinase activity did not change markedly during liver development. After partial hepatectomy GMP synthetase activity was elevated, reaching a peak of 155% of the sham-operated control values by 36 h after the operation. GMP kinase activity was not affected by partial hepatectomy. After 3 days starvation hepatic GMP kinase activity decreased slightly faster than total cytosol protein, while GMP synthetase activity was preferentially maintained. These results indicate that GMP synthetase activity was linked with cellular proliferation in differentiating, regenerating and neoplastic tissues.