SYNOPSIS. In order to understand development it is necessary to elucidate the mechanisms behind both gene regulation and cell type determination. I have approached these problems in three ways utilizing the amphibian system. The first involves the injection of somatic cell nuclei into oocytes. The oocytes are used as a coupled transcripuonal-translational system to detect the translational products of the injected nuclei. In this way it becomes possible to examine the regulation of genes coding for ubiquitous or housekeeping functions as well as those coding for differentiated functions. The results indicate that in the case of the proteins examined, the oocyte cytoplasm appears to be able to regulate the synthetic activity of the transferred nuclei so as to conform to its own synthetic output. A second approach involves the examination of the expression of various classes of genes in interspecific amphibian hybrids. These studies show that there appears to be preferential expression of the maternal allele of a differentiated product, alcohol dehydrogenase, while the maternal and paternal alleles of ubiquitous products such as lactate dehydrogenase and superoxide dismutase are expressed simultaneously. These results suggest that there may be differences in the regulation of various classes of genes. The third approach involves the injection of bacterial plasmids containing eukaryotic genes into amphibian oocytes. A plasmid, pSp2, containing sea urchin histone genes HI, H2b, and H4 was injected into the germinal vesicle of Xenopus laevis oocytes. The genes coding for sea urchin histones HI and H2b were expressed. The applicability of this system to problems of gene regulation and cell type determination is discussed.