Small nuclear RNAs (snRNAs) and small nuclear ribonucleoprotein particles (snRNPs) were purified from the cat placenta as an initial step in the analysis of the regulatory mechanism(s) of gene expression in the placenta. For visual observations, snRNA species including 7S, U2, U1, U4, X-I, X-II, 5S, U5, U6 and X-III were stained with acrydine orange, following the extraction of RNAs with phenol from cat placental cell nuclei and their fractionation by electrophoresis on a 10% polyacrylamide gel containing 7 M urea. The small nuclear RNA U1 from the cat placenta showed approximately the same mobility as that of mouse (129/sv) liver used as a reference on the gel. The logarithms of the nucleotide lengths of cat placenta snRNAs were inversely related to their mobilities. The approximate number of snRNA molecules in the cat placenta cell nucleus was also calculated. Highly purified U1-snRNP which comprises U1 snRNA and eight polypeptide components ranging in molecular weight from 68, 000 to 9, 500 was immunochemically purified using systemic lupus erythematosus-anti(U1)RNP antibody column chromatography. Any polypeptide component involved in the U1-snRNP did not immunoreact with normal human IgG. No immunoreactive materials with anti(U1)RNP antibodies could be detected in cat amniotic fluid. The U1-snRNP and U2-snRNP rich fractions were separated biochemically on a DEAE Sepharose column.