AbstractCyclic GMP (cGMP)‐binding cGMP‐specific phosphodiesterase (PDE5) activity was found in chick dorsal root ganglia (DRG). PDE5 expression was studied at different stages of development: in embryonic day 10 (E10) and E18 embryos and in 5‐day post‐hatching chick (P5). The presence of PDE5 was suggested by the ion exchange chromatography elution profile in E18 DRG extracts, where cGMP‐specific hydrolytic calmodulin‐independent activity was found; in other stages, this activity coeluted with the PDE1 calmodulin‐stimulated isoform characterized previously. Inhibition studies supported the hypothesis that the newly identified PDE activity belongs to the PDE5 isoform. Western blot analysis using a PDE5‐specific antibody was also carried out and revealed the presence of three specific immunoreactive bands with apparent molecular weights of 98, 93, and 86 kDa, corresponding to the three described splice variants (PDE5A1, PDE5A2, and PDE5A3). The expression in DRG of the three PDE5 isoforms was also confirmed by RT‐PCR. Developmental regulation of PDE5 was revealed by the immunoblot analysis at different stages; expression was very low at E10 but an overall substantial increase occurred between E10–18 (about 12‐fold, considering the three PDE5 isoforms together). Differences were revealed, however, when a single PDE5 isoform was considered. PDE5A1 and PDE5A3 showed an increase at all stages although more pronounced between E10–18, whereas PDE5A2 underwent a marked increase (about 38‐fold) in the first period and remained nearly constant between E18 and P5. This is the first evidence of PDE5 in sensory neurons, and the distinct temporal expression patterns of enzyme isoforms may indicate different physiologic roles in developing and mature chick DRG. © 2004 Wiley‐Liss, Inc.