In ball milling the rates of breakage vary primarily with the size distribution of powder in the mill. Using an approximate solution to the batch-grinding equation, the acceleration and deceleration of breakage rates of all individual size classes are determined when the grinding environment is varied. Experimental results with limestone and copper ore show time-dependent breakage rates under both dry and wet grinding conditions. The effect of grinding additives on breakage rates is also studied. In general, breakage rates increase for coarse sizes and decrease for fine sizes with an increase in the fines present in the mill. Such variations have strong implications in closed-circuit grinding.