This work characterizes the development of the saxitoxin (STX)-sensitive Na+ channels from rat whole forebrain between embryonic day 15 (E15) and postnatal day 90 (P90), both with binding studies and with single channel studies. The Na+ channel total mRNA and the individual mRNAs encoding Na+ channels I, II and III were also determined. The total STX binding rose about 40-fold from E15 to reach a plateau at P30 and its temporal course correlated with the expression of Na+ channel total mRNA. Low affinity and high-affinity STX binding sites, predominant in embryonic and postnatal forebrains, respectively, were found. The single channel studies of batrachotoxin-modified channels also revealed two main populations. In E15 only low-affinity channels (K-D = 32.7 nM; 200 mM NaCl) and in P30 only high affinity ones (K-D = 1.6 nM) were present. At P0 channels with intermediate affinity (K-D range 3-34 nM) were observed. The increase in affinity was due to a gradual increase in the STX association rate.