Extensive research work has been conducted by the writers on the effects of barite, thinners, and fiocculants (in various concentrations) on freshwater bentonite muds. The PV-----versus-VP----curves were used to evaluate these effects. Definite, characteristic regularities were observed. Experiments have also been performed on salt-saturated atta-pulgite (Zeogel) muds on addition of various concentrations of pregelatinized starch and carboxymethylcellulose. The results were evaluated in terms of slope and location of PV----versus-yp lines. The yp/PKratio (i.e., slope of the PK----versus-yp line) is a significant indicator of drilling fluid condition. The P K---versus-yP curves can be used for evaluating the stability of drilling fluids. They can also serve as guidelines in the field control of the drilling fluid systems and in taking timely actions.