The eΓΎ ects of formic acid, three concentrations of formaldehyde in a formic acid/ formaldehyde mixture and cysteine-peptidase inhibitors, 1-trans epoxysuccinyl-leucylamido-(4guanidino) butane (E-64), N-ethylmaleimide and cystamine on nitrogen (N) distribution during ensilage of perennial ryegrass (Lolium perenne) were investigated. A third cut or perennial ryegrass (163 g dry matter kg-1 and 61 g water-soluble carbohydrate kg-1 dry matter) was ensiled in two silo sizes ; formaldehyde-treated herbage was ensiled only in larger silos (500-550 g herbage) and cysteinepeptidase inhibitor-treated herbage only in smaller silos (130-150 g herbage). Control silages were poorly fermented but contained low concentrations of butyric acid and ammonia N indicating little activity of spoilage bacteria. Formic acid increased peptide N concentrations (P AE 0.01) in silage from smaller silos but had little eΓΎ ect on other N constituents ; in the larger silos, formic acid reduced soluble non protein nitrogen (NPN) and ammonia-N concentrations and increased peptide N concentrations. Increments in formaldehyde reduced silage soluble and ammonia N concentrations (linear eΓΎ ect ; P AE 0.001). N-Ethylmaleimide and E-64 reduced soluble NPN concentrations (P AE 0.05) but had little eΓΎ ect on other N constituents. Cystamine, however, increased silage peptide N concentrations. Gel ΓΌltration on Sephadex G-25 of silage juice prepared from control and formic acid-treated silages suggested that most silage peptides were small, with molecular weights of less than 520 Da.