Reaction of the first excited singlet state of ethidium with CU2+ ions results in electron transfer to form oxidized ethidium and Cu+. The decay of these species is via reverse electron transfer, leaving the system unchanged. The reverse electron transfer reaction has been studied in solutions containing polynucleotides, leading to information concerning binding of Cu+ and electron transfer in these media. Cu+ binds strongly to calf thymus DNA, poly(deoxyadenylic-thymidylic acid), and poly(deoxyguanylic-deoxycytidylic acid), in agreement with previous work. The rate of escape of Cu+ from the poly(deoxyadenylic-thymidylic acid) helix is measured as (1.3 +/- 0.3) x 10(5) s(-1). The strongest complex is that with poly(deoxyguanylic-deoxycytidylic acid), and in this case reverse electron transfer occurs over several decades of time. Fitting these data to a summation of exponentials, k(et) = Sigma nu exp(-beta r), where r is the separation distance quantized in base pairs, leads to an attenuation factor, beta, of 0.73 +/- 0.05 Angstrom(-1), for electron transfer in this medium.