Many butterflies acquire nutrients from non-nectar sources such as puddles. To better understand how male Papilio butterflies identify suitable sites for puddling, we used behavioral and electrophysiological methods to examine the responses of Japanese Papilio butterflies to Na(+), K(+), Ca(2+), and Mg(2+). Based on behavioral analyses, these butterflies preferred a 10-mM Na(+) solution to K(+), Ca(2+), and Mg(2+) solutions of the same concentration and among a tested range of 1 mM to 1 M NaCl. We also measured the ion concentrations of solutions sampled from puddling sites in the field. Na(+) concentrations of the samples were up to 6 mM, slightly lower than that preferred by butterflies in the behavioral experiments. Butterflies that sipped the 10 mM Na(+) solution from the experimental trays did not continue to puddle on the ground. Additionally, butterflies puddled at sites where the concentrations of K(+), Ca(2+), and/or Mg(2+) were higher than that of Na(+). This suggests that K(+), Ca(2+), and Mg(2+) do not interfere with the detection of Na(+) by the Papilio butterfly. Using an electrophysiological method, tip recordings, receptor neurons in contact chemosensilla inside the proboscis evoked regularly firing impulses to 1, 10, and 100 mM NaCl solutions but not to CaCl(2) or MgCl(2). The dose-response patterns to the NaCl solutions were different among the neurons, which were classified into three types. These results showed that Japanese Papilio butterflies puddle using Na(+) detected by the contact chemosensilla in the proboscis, which measure its concentration.