Insulin-like growth factor II (IGF-II) is present at high levels in fetal and early neonatal rat plasma, and decreases profoundly following birth. In the present study, the levels of IGF-II RNA in different rat tissues at different ages were determined by hybridization to a rat IGF-II cDNA probe. IGF-II RNA was present in 11 of 13 fetal or neonatal tissues examined: at higher levels in muscle, skin, lung, liver, intestine, and thymus; at lower levels in brain stem, heart, cerebral cortex, kidney, and hypothalamus; and undetectable in spleen and pancreas (although the latter RNA was partially degraded). In each tissue, Northern blot hybridization revealed the presence of six IGF-II RNAs: 6, 4, 3.8, 2.2, 1.7, and 1.2 kilobase pairs, consistent with results previously observed in the BRL-3A rat liver cell line and attributed to alternative RNA processing. Although differences in the relative abundance of these RNAs were observed in different tissues, the same size species occurred in all tissues with the 4-kilobase pair RNA the most abundant species. RNAs from the different tissues were examined at six developmental ages (days 16 and 21 of gestation; days 2, 11, 22, and 75 after birth) by hybridization to slot blots and Northern blots. In lung, thymus, kidney, and brain stem, IGF-II RNA was expressed at higher levels in the fetus than after birth, whereas in muscle, skin, liver, heart, and intestine, the high fetal levels of IGF-II RNA continued through day 11 or day 22 after birth. IGF-II RNA persisted into adulthood in cerebral cortex and hypothalamus. Although the significance of these tissue-specific differences in the developmental regulation of the expression of IGF-II RNA remains to be established, they exhibit intriguing temporal correlations with major maturational events in some tissues such as lung and muscle.