Cats and Oranges

After a brief, but pleasant, interlude, Sam has resumed his habit of inappropriate excretion. (Solids are easier to remove than liquids.) Someone recommended sprinkling the affected area (carpet not cat) with orange oil -- but I've just read that essential oils can be harmful to pussycats. Any other ideas?

I don't think it's stress: he stopped doing it during the month he should've been most stressed! And it doesn't seem to have any relation to cleanliness of litter tray which is in the same bloody room, he has to walk past it to pee under the sink or to presence of others. And I hate terrorising him, though if he gives me that exasperated look again I shall take a photo.