Two non-steroidal anti-inflammatory commonly used in Mexico are paracetamol and diclofenac. These drugs reach natural waterbodies transport through the hydrological areas of agriculture and effluent treatment plants and can affect plankton. In this study, we quantified the chronic effects of paracetamol and sodium diclofenac on the population dynamics of the rotifer Plationus patulus and the cladoceran Moina macrocopa under laboratory conditions. Both these species of zooplankton are common in freshwaters and are sensitive to chemicals in the medium. Our results showed that densities of both P. patulus and M. macrocopa decreased with increasing levels of both the analgesic drugs. Both zooplankton species did not survive beyond when paracetamol was applied at 32 mg I(-1) in the medium. Diclofenac in general had more adverse effect than paracetamol for either zooplankton species. P. patulus was more sensitive than M. macrocopa to both analgesic drugs. When diclofenac was present in the medium at > or = 12.5 mg I(-1), rotifer reproduction was inhibited, while the tested cladocerans continued to grow but to lower densities compared to control. The rate of population increase (r) per day of P. patulus and M. macrocopa was significantly and inversely related to the concentration of paracetamol and diclofenac in the medium. However, the relationship between r and drug concentration differed depending on the zooplankton species and drug. In controls, the r of P. patulus was 0.18 d(-1), for M. macrocopa under similar conditions, it was slightly lower (0.16 d(-1)). The rvalues of both zooplankton populations became negative (-0.10 to -0.15 d(-1)) when exposed to paracetamol at 32 mg l(-1) or diclofenac at 25 mg I(-1).