Nitrogen retention (N_ret) and denitrification were studied in the mesotrophic Solina reservoir (south-east Poland) between spring and autumn in 2002 and 2003. N_ret was calculated on the basis of the input-output mass balance. The rate of denitrification was measured using the ¹⁵N isotope pairing technique in two types of areas; in deep areas, where the bottom sediment makes contacts with the hypolimnion (D_prof), and in the unstratified shallows (D_litt). The calculated daily load of removed nitrogen (D_tot) varied somewhat from 11.0 to 19.1 mg N m^–2 day^–1 with a standard deviation of 3.0 mg N m^–2 day^–1. The limited oscillation noted for D_tot results from the major participation of the deep areas of the reservoir, which are characterised by stable D_prof, as distinct from D_litt, which varied seasonally from 5.8 mg N m^–2 day^–1 in November to 109.9 mg N m^–2 day^–1 in August. The primary factor limiting the denitrification rate was temperature, the other being availability of substrates, mainly organic matter. Nitrogen retention ranged seasonally between –25.0 and 175.9 mg N m^–2 day^–1. The contribution of denitrification to N_ret ranged from 9.0% to 49.8% (mean 16.4%). On average, only 4.9% of the nitrogen external load was denitrified (range 2.6–7.9%, s.d. 1.5%). After comparing obtained data with others available in the literature, we concluded that reservoirs, estuaries and highly-loaded lakes in a river system experience a more limited contribution of D_tot to N_load than is the case in natural lakes.