Heat and salt balances are estimated over the northern California shelf from early December 1988 through late February 1989 (winter) and from early March through early May 1989 (spring) from moored meteorological and oceanographic time series taken in 93 m of water 6.3 km from the coast. We find a winter mean offshore heat flux of 8.7¿105 W m-1, about a factor of 5 smaller than earlier estimates of the mean summer (upwelling season) offshore heat flux on the northern California shelf. The mean offshore heat flux is predominantly in the surface boundary layer and is balanced by an along-shelf heat flux divergence (as represented by an eddy along-shelf temperature gradient flux) and a cooling trend making the mean winter heat balance fundamentally three dimensional. In contrast to winter, the spring mean offshore heat flux of 6.4¿105 W m-1 is balanced by a positive air-sea heat flux of 8.3¿105 W m-1, which is about 80% of the mean air-sea heat flux in summer. This makes the spring mean heat budget primarily two dimensional, like the summer mean heat budget off northern California. On timescales of days the dominant terms in the fluctuating heat budget in both winter and spring are the cross-shelf heat flux and local changes in heat content. These are well correlated with each other and with the local along-shelf wind stress. The along-shelf temperature gradient flux, uncorrelated with the along-shelf wind stress, is usually weak on timescales of days. Occurrences when it is strong are interpreted as effects of mesoscale features. Mean and fluctuating cross-shelf salt fluxes provide essentially the same information as cross-shelf heat fluxes. This is not surprising in light of the strong temperature-salinity relationship on the northern California shelf. ¿ American Geophysical Union 1994