Nucleoplasm, also called nuclear sap or karyoplasm, is the fluid usually found in the nucleus of eukaryotic cells. This fluid contains primarily water, dissolved ions, and a complex mixture of molecules. Its primary function is to act as a suspension medium for the organelles of the nucleus. Other functions include the maintenance of nuclear shape and structure, and the transportation of ions, molecules, and other substances important to cell metabolism and function.

There are many types of plasm contained within a eukaryotic cell. The protoplasm is all of the cell's contents, including the cytoplasm and nucleoplasm. Cytoplasm includes all cell fluids and organelles within the cell membrane or wall, but outside the nucleus. Cytosol, a fluid similar to nucleoplasm in both function and composition, is the main component of cytoplasm, making up about 70% of the cell's total volume. A nuclear membrane around the cell's nucleus separates the nucleoplasmic fluid from the cytoplasm in the rest of the cell.

The highly viscous nucleoplasmic fluid suspends and protects the nucleolus. This organelle is composed of proteins and nucleic acids. It is responsible for the transcription and assembly of ribosomal ribonucleic acid (rRNA), a type of RNA that works together with messenger ribonucleic acid (mRNA) and transfer ribonucleic acid (tRNA) to transform amino acids into proteins.

Also suspended in the nucleoplasm is a complex substance known as chromatin, which condenses during cell division to form chromosomes. Chromatin is primarily made up of deoxyribonucleic acid (DNA) and proteins called histones. When cell division occurs, the nuclear membrane dissolves and the nucleus releases its nucleoplasm, along with the genetic information contained in the fluid. After the process of division is complete, the two resulting sister cells reform their nuclear membranes, allowing their nuclei to refill with nucleoplasmic fluid.

Nucleotides are also dissolved in the nucleoplasm. These molecules are one of the building blocks for both RNA and DNA. Enzymes that direct the activity taking place in the nucleus can also be found dissolved in nucleoplasmic fluid. The fluid allows for easy transportation of these molecules and enzymes within the nucleus. As the nuclear membrane is impermeable to many types of molecules, protein complexes in the membrane, called nuclear pores, allow substances to be transported between the cytoplasm and nucleoplasm.

The nuclear matrix is also thought to be contained in the nucleoplasm. However, the form, function, and even the existence of the nuclear matrix are subjects of debate. Generally, it is believed that the nuclear matrix is a network of protein fibers responsible for maintaining the size and shape of the nucleus. If this is true, then the nuclear matrix is similar in function to the cytoskeleton found in the cell's cytoplasm.