A coupled assay using [14C] 4-aminobutyric acid and a direct assay using [14C]succinic semialdehyde have been designed to assay the activity of succinic semialdehyde dehydrogenase in a patient with 4-hydroxybutyric aciduria and family members. In the coupled assay less than 3% of control succinic semialdehyde dehydrogenase activity was found in lysates of lymphocytes isolated from whole blood of the patient. In the direct assay there was no detectable activity of the enzyme in lysates of isolated lymphocytes or cultured lymphoblasts. Results indicated the parents to be heterozygous carriers of the abnormal gene, consistent with an autosomal recessive inheritance.