We previously showed that at pH 7.4, the cytosol and a low speed pellet (3300 X g) of rat ventral prostate degraded rat PRL, whereas a high speed pellet (25,000 X g) was inactive. The current study further explores PRL degradation by ventral prostatic tissue. Enzyme markers indicated that the low speed pellet was mitochondria enriched (cytochrome c oxidase), and the high speed pellet was lysosome enriched (acid phosphatase), although there was considerable cross-contamination between the two fractions. Proteolysis of hormone was examined by incubating 125I-labeled rat iodo-PRL with tissue fractions, followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and radioautography to identify products and quantify the extent of PRL degradation. Acidification of the incubation medium (pH 4-5) to optimize lysosomal protease activities inhibited PRL degradation in cytosol and activated the process in both pellets consistent with the distribution of acid phosphatase. Under acidic incubation conditions, both low and high speed sediments produced the same electrophoretic pattern of PRL degradation, indicating the generation of peptide fragments from the hormone with a molecular mass of approximately 16,000-8,000 daltons. Cytosol (pH 7.4) also produced peptide fragments in this weight range, although at much lower rates and in relatively lower proportions. Peptide fragment formation from PRL by the low speed pellet (pH 7.4) was minimal. On a protein basis, PRL degradation by the high speed sediment (pH 4.5) was 3-10 times that by the low speed sediment (pH 7.4). Enzyme inhibitor analysis indicated differences in each of the cell fractions with regard to the types of proteases involved in PRL degradation. When compared with other tissues of the male rat, ventral prostate was the most active in degrading PRL and generating peptide fragments. In addition, kidney, spleen, and lung were among the more active tissues, whereas liver, was deferens, and dorsolateral prostate were relatively less active in degrading hormone. Our studies indicate that qualitatively different processes degraded PRL in each of the active subcellular fractions and that the generation of peptide fragments from PRL predominates in a lysosome-rich fraction of ventral prostate. The possible significance of subcellular variations in PRL processing and the generation of peptide fragments of the hormone by peripheral tissue are discussed.