Methylglyoxal bis(guanylhydrazone) (MGBG) was incorporated extensively into baby hamster kidney (BHK-21/C13) cells by a temperature - and Na⁺, K⁺ - ATPase linked mechanism. The uptake of the drug was dependent on the growth state of the cells and transformed cells (PyY-cells) incorporated MGBG to a greater extent than normal BHK cells. The uptake of the drug was inhibited by coadministration with spermidine, spermine and N¹-acetylspermine and to a lesser extent by putrescine and Mg²⁺ ions. MGBG and spermine were cytotoxic and both compounds stimulated spermidine acetyltransferase (SAT) activity in BHK cells but not in PyY-cells suggesting that transformation enhanced resistance of the cells.SAT activity was present in both the nuclear and cytosolic compartment of both cell-lines and MGBG stimulation resulted in an increase in both N¹- and N⁸-acetylspermidine production in the nucleus and almost exclusive N¹-acetylspermidine production in the cytosol. MGBG stimulated spermine acetyltransferase activity in both nuclear and cytosolic compartments of mammalian cells. Increased intracellular acetylation after MGBG-treatment resulted in enhanced excretion of putrescine and N¹-acetylspermidine from both BHK- and PyY-cells implicating acetylation as a means to control intracellular polyamine content. MGBG was itself acetylated in the nucleus of both cell-lines, as was putrescine. However, prior MGBG treatment inhibited the acetylation of MGBG itself and the diamine suggesting that the two compounds are acetylated by a common nuclear acetyltransferase.A cytosolic spermidine acetyltransferase enzyme was purified over 2000-fold from BHK-cells and was found to acetylate putrescine, spermidine, spermine and MGBG. The activity of the enzyme preparation to all these compounds suggests that the enzyme preparation contained nuclear acetyltransferase activity. A model is preposed for the role of acetylation in both the nuclear and cytosolic compartments of mammalian cells.