Urea regeneration by size-fractionated plankton was measured over an annual cycle at a coastal station in the permanently well-mixed waters of the western English Channel. Rates of urea regeneration in the < 200 mu m fraction varied from 0.6 to 20.6 nmol N L-1 h(-1). Regeneration rates were lowest in winter and highest in summer. The ratio of the rates of regeneration to uptake of urea was close to 1 on all time (seasonal and nycthemeral), and space (vertical) scales indicating that regeneration by microheterotrophs supplied the totality of urea used by phytoplankton. On an annual basis, urea regenerated by the microheterotrophs (0.98 mol N m(-2) year(-1)) was equivalent to similar to 33% of the total regenerated N (urea + ammonium). The major part of urea regeneration was due to the nanoplankton (51%) and microplankton fractions (36%). Regeneration of urea in the picoplankton was detectable only from April to October and represented, on an average, 25% of the total urea regenerated during this period. Urea regeneration in micro- and nanoplankton fractions was mainly associated with ciliates and in the picoplancton fraction with bacteria.