In mammals, including most human populations, intestinal lactase activity is very high in the suckling and declines to low levels after weaning. There are two lactase phenotypes among human adults, one with persistent high lactase activity and one with hypolactasia(1). In the present study lactase mRNA levels have been measured by cDNA probes on northern blot of poly(A)+RNA normalized by B-actin in the small intestine of raboit, at different ages, and on slot blots of total RNA and polyiA)+RNA of human adults with persistent high lactase activity and with hypolactasia. An ECO RI-Hind III fragment covering more than the 5'-half of the rabbit lactase cDNA and the full human lactase cDNA were used as probes (2). mRNA levels have been compared with those of lactase activity (3). In the rabbit mRNA appears by the end of the gestational period and a level, comparable to that found in the suckling animal, is still present in the adult life. The lactase activity appears at the same time of the mRNA, but it declines to very low levels postweaning. Similarly, in man, at RNA level, no clear differences were found between adults with hypolactasia and adults with persistent high lactase activity. Conclusion: In adult mammals and in adult human hypolactasia, the control of lactase gene expression is very likely to be at a post-transcriptional level. 1) Auricchio S. et al. Lancet 2,324-326, 1963; 2) Mantei N. et al. The EMBO J.7, 2705-2713, 1988; 3) Asp NG. et al. Anal. Biochem. 47,527-538,1972.