The standard height rules applied in off-route airspace are examined to assess the degree of intrinsic safety they provide, i.e. the reduction of conflicts without action being taken by pilots or ATC. The yardstick used is the conflict rate which would obtain if the aircraft were uniformly randomly distributed in the height dimension and flying straight and level on uniformly randomly distributed tracks. It is shown that the application of the standard rules can lead to a reduction in intrinsic safety unless significant height-keeping errors are present. An alternative height rule apparently having more desirable characteristics is examined on the same basis.