A developmental study of myelin basic protein (MBP) variants in eight regions of pig nervous system (NS) was performed using a quantitative electroimmunoblotting procedure. Four major MBP forms with apparent molecular weights of 21.5K, 20.2K, 18.5K, and 17.3K were identified in both the CNS and the PNS and were detected as early as 22 days before birth. Quantification of the most abundant forms, the 21.5K and 18.5K MBPs, revealed characteristic profiles of accumulation of these two variants in different regions of the NS. The ratio of 21.5K:18.5K MBP varied with developmental time as well as with the various NS regions, peaking 20 days postnatally. The 17.3K MBP was observed from embryonic stages to adulthood, as were the 21.5K and 18.5K forms. In contrast, the 20.2K variant appeared most abundant from 10 days before to 22 days after birth and thereafter decreased in intensity so as to be no longer detectable in the brain of a 5-year-old pig. A similar pattern was also observed with an anti-MBP-reacting protein with an apparent molecular weight of 23K. Taken together, these results suggest that in the pig NS, the expression of MBP variants may be regulated both regionally and developmentally.