Inward rectification of cardiac I_K1channels was modulated by genetic manipulation of the naturally occurring polyamines. Ornithine decarboxylase (ODC) was overexpressed in mouse heart under control of the cardiacα -myosin heavy chain promoter (α MHC). In ODC transgenic hearts, putrescine and cadaverine levels were highly elevated (≡35-fold for putrescine), spermidine was increased 3.6-fold, but spermine was essentially unchanged. I_K1density was reduced by ≡38%, although the voltage-dependence of rectification was essentially unchanged. Interestingly, the fast component of transient outward (I_to,f) current was increased, but the total outward current amplitude was unchanged. I_K1and I_tocurrents were also studied in myocytes from mutant Gyro (Gy) mice in which the spermine synthase gene is disrupted, leading to a complete loss of spermine. I_K1current densities were not altered in Gy myocytes, but the steepness of rectification was reduced indicating a role for spermine in controlling rectification. Intracellular dialysis of myocytes with putrescine, spermidine and spermine caused reduction, no change and increase of the steepness of rectification, respectively. Taken together with kinetic analysis of I_K1activation these results are consistent with spermine being a major rectifying factor at potentials positive to E_K, spermidine dominating at potentials around and negative to E_K, and putrescine playing no significant role in rectification in the mouse heart.