The influence of a 7.7-mumole (0.5-mg) dose of parenteral zinc on the synthesis of metallothionein in rat kidney was examined. The amount of zinc bound to metallothionein was maximal 9--15 hours after zinc administration. Pulse labeling with [35S]cystine showed the rate of renal metallothionein was stimulated to a maximum 6 hours after zinc administration and declined thereafter. Total RNA was extracted by the guanidine thiocyanate procedure from kidneys that had been flash frozen in liquid N2. Polyadenylated RNA (mRNA) was isolated by oligo (dT)-cellulose chromatography. The mRNA was translated in a wheat germ system and newly synthesized metallothionein was isolated by activated thiol--Sepharose 4B chromatography. Metallothionein mRNA activity nearly doubled after zinc administration adn was closely correlated with the enhancement in synthetic rate of this protein in kidney. Actinomycin D administered prior to either zinc or cadmium completely blocked both the stimulation of metallothionein synthesis and mRNA activity found in kidney in response to these metals. The results suggest renal metallothionein is induced by zinc or cadmium through a mechanism that requires altered expression of the metallothionein gene(s).