Previously we reported that the rat B-50/GAP-43 gene contains two promoters (P1 and P2). This study describes the contribution of these two promoters to the mRNA population in several paradigms leading to an altered B-50 mRNA expression. In 8-day-old rat brain we found that P1 transcripts (1676 +/- 50 nt) account for 5% and P2 transcripts (1462 +/- 46 nt) for 95% of the B-50 mRNAs. The expression of P1 and P2 derived transcripts is high at postnatal day 8 and the ratio between the amount of transcripts derived from P1 and P2 did not change during (embryonal and postnatal) development or aging. After peripheral nerve crush or transection B-50 mRNA expression in induced in the distal nerve stump. The amount of transcript in the nerve stump distal of the lesion derived from both P1 and P2 was increased and the ratio between P1 and P2 transcripts was not altered. To determine whether both P1 and P2 transcripts are translated, a polyribosomal profile from 8-day-old rat brain was generated. Northern analysis showed that both transcripts were associated with approximately four ribosomes. Since no change could be found in the activity in either of the two promoters under the different circumstances tested, we conclude that the activity of the two rat B-50 gene promoters is regulated by a similar mechanism.