Abstract: Toxicity measuring apparatus comprising a sample reservoir in which an aqueous sample to be tested is stored, a reagent reservoir, a diluent reservoir, a mixing chamber vented via a first valve to atmosphere through an air inlet tube which extends within the chamber towards a lower part thereof, second, third and fourth valves via which the sample, the reagent and the diluent respectively are fed selectively to the mixing chamber in accordance with the open/closed state of the said second, third or fourth valves, a toxicity detector cell fed with a mixture of the sample, the reagent and the diluent from the chamber, within which detector cell aqueous sample measurements are carried out, a linear pump and valve means, via which valve means the mixture is drawn through the detector cell from the chamber by the linear pump, or via which air in an upper part of the chamber is drawn from the chamber by the pump in accordance with the open/closed state of the said valve means, whereby for the purpose of mixing the