I see slight edge burn. Might be too much nitrogen. Otherwise it looks normal to me.
Sey

Could also be environmental issue. BTW did you harden it off first before bringing it out? Also, what is your mix made up of?

What were the overnight lows? Maybe it was a little chillier outside but looks like nothing to worry about.

Thanks for your ideas. But about 8 hours later, except for the stem and first two seed-leaves, the rest of it is all either droopy or curled in. It looks all but dead. I guess I'll have to start again with this pot (the other one looks fine)

The soil is a planting mix I had very good experience with in the past, temperatures are very nice (actually we have a 40°C Heatwave coming in today, I might bring them back in for a day or two). I didn't do any hardening - there is only a slight difference between the inside and the outside.

Only grasping at straws here, but your mix looks possibly very water retentive and heavily peat based. You might have had good results in the past using it with container tomatoes, but not now, which is why I asked you the make-up of your mix, but since you are sure it's not the problem.........

....also, unless your plants are getting no sun outside, hardening off is a MUST before bringing a tomato plant outdoors, or it will stress. But you say that's not it either.

I don't think the potting mix would make it go downhill overnight. Even if the temps are similar indoors and out, the plant still has to get used to direct sun and wind. So I'd agree perhaps it's plain old transplant shock.

"I don't think the potting mix would make it go downhill overnight."

Depends what's in it :-) But yes, I generally agree. There are proven mixes that work. I use the 5-1-1 mix described over on the containers forum, and have been overwhelmed with tomatoes ever since.

edweather, you make a good point that if there's some kind of contamination in the soil, herbicide for instance, it could affect the plant quickly.

I also use a form of 5:1:1, and have much better results than with potting soil. In fact right now, I have some very happy strawberries in it. The pot next to it has some potting soil and the strawberry plants are dying. But my point is, it did take longer than overnight for that to happen.

We're having a crazy heatwave here, and I made some very poor judgement of leaving both pots outside. Although they were watered properly, both plants dried out totally. Lesson learned.

Luckily I kept some more plants indoor, so once the heatwave is over I'll do proper hardening - how many days would you recommend outside before I can transplant again?