The normal developmental profile of preprotachykinin (PPT) gene expression was determined in rat striatum from embryonic day 20 to adulthood (greater than 45 days of age). At embryonic day 20, the amount of striatal PPT mRNA was approximately 8% of adult levels, and this specific mRNA approached adult levels by postnatal days 12-15. The absolute amount of PPT mRNA, determined by comparison with PPT mRNA synthesized in vitro, ranged from 0.2 pg/micrograms of total RNA at embryonic day 22 to 5-6 pg/micrograms of total RNA in adult striata. In addition, the proportion of the various PPT mRNAs derived from the same primary transcript by alternate splicing was determined in the developing animal. At those ages at which PPT mRNA levels were significantly less than those in the adult, there was a slight (10%) but statistically significant increase in the relative amount of gamma-PPT mRNA with respect to the amount of beta-PPT mRNA. Because these mRNA species encode different combinations of tachykinin peptides, these data suggest that the proportion of substance P versus various neurokinin A-related peptides may be developmentally regulated.