microviscosity of hydroxypropylcellulose gels as a basis for prediction of drug diffusion rates this study investigated the influence of the rheological properties of hydroxypropylcellulose hpc gels on the in vitro release of theophylline included in the gel at 02 gl experiments were performed with six hpc varieties mean molecular weight between 5x105 and 12x106 nominal viscosity between 100 and 4000 mpas at concentrations of 02 ww theophylline diffusion coefficients at 37 degrees c ranged from 35x107 to 11x103 cm2min and were in all cases markedly higher than those predicted on the basis of gel macroviscosity as determined by capillary viscometry in general the theophylline diffusion coefficient declined exponentially with hpc concentration in the case of the lowestmolecularweight hpc however the diffusion coefficient remained constant to hpc concentrations of up to 08 probably because of the high entanglement concentration of the hpc gel microviscosities as determined by dynamic light scattering dls with latex microspheres 162 nm diameter were considerably lower than the macroviscosities determined by capillary viscometry and similar to microviscosities estimated on the basis of theophylline diffusion nevertheless macroviscosity was correlated with microviscosity suggesting that it is of value for approximate estimates of rates of diffusion of theophylline from hpc gels