Mouse parotid acinar cells differentiate and proliferate mainly after birth. During the first 3 weeks of age, α-amylase mRNA, one of the major gene products of the adult tissue, increases from barely detectable to adult levels (10⁴ copies/cell). Run-on transcription experiments show that this increase is transcriptionally regulated. Northern blot hybridization and in situ hybridization results indicate that the two promoters of the α-amylase gene Amy-1a are differentially switched on. First, the weaker downstream promoter is activated, and by 2 weeks of age, virtually all acinar cells have accumulated the transcript initiated at this promoter. At this age the strong Amy-1a promoter is utilized in only a minor proportion of acinar cells, while in the adult this promoter appears to be active in all acinar cells. Thus, the progressive accumulation of α-amylase mRNA during postnatal parotid differentiation is mainly the consequence of progressive acinar cell commitment to expression of the strong parotid-specific Amy-1a promoter. The pattern of committing cells during differentiation suggests that once an acinar cell has initiated expression of parotid-type α-amylase mRNA, this commitment is passed on to its daughter cells.