Northern blot analysis determined whether multiple alpha 4 transcripts for neuronal nicotinic receptors in rat brain could be detected as distinct bands. When poly(A)+ RNA was isolated from brain regions and hybridized with a Hinfl fragment of alpha 4-1 cDNA containing a sequence shared by both alpha 4-1 and alpha 4-2, but little homology with other alpha or beta subunits, bands at 6.0, 4.6, and 2.6 kb were obtained. When a Taql fragment with selectivity for alpha 4-1 was used, a single band was present at 6.0 kb. The 6.0-kb band was least abundant in all brain regions; the 2.6-kb band was most abundant in frontal cortex, hippocampus, striatum, basal forebrain, and thalamus, whereas the 4.6-kb band was most abundant in midbrain and cerebellum. Nicotine (3.6 mumol/kg, a.c., twice daily) increased the abundance of the 4.6-kb transcript in frontal cortex significantly by 28% following 2.5 days of injections; the 6.0- and 2.6-kb transcripts were unchanged. Nicotine did not affect alpha 4 transcripts in other brain regions. Results suggest that increased mRNA levels may mediate the nicotine-induced up-regulation of receptors in cerebral cortex.