The chemistry for sodium at mesospheric heights has been adapted for mesospheric potassium. The profile computed for potassium agrees well with mid-latitude lidar observations. A nightglow of 1 R is estimated for 767 nm, an amount probably undetectable. Both lidar data and photometer data indicate that the seasonal variation of potassium, unlike that of sodium, is negligible. Their different seasonal patterns are argued to arise most likely from modest differences in the rate coefficients, especially their dependence on temperature, for comparable reactions. Thus, the computed seasonal behavior of Na better matches the data if an exponential temperature dependence is assumed for NaO2+O→NaO+O2, but a similar assumption for potassium leads to poorer agreement with data. The twice larger KO2 versus NaO formation rate appears responsible for the fact that variations of atomic oxygen alter the total abundance of sodium more than they do for potassium. Eddy transport is apparently also important to the seasonal variation of Na, influencing both the total sodium species profile and the atomic oxygen distribution. Potassium and sodium concentrations near and below the peak appear unaffected by ion chemistry. This is probably less true of lithium. ¿American Geophysical Union 1987