Only one picture of this weedy looking plant in the rainforest very close to a river. If I remember well the ripe fruit (2-2,5cm diameter) was orange red, the rather large leaves have long spikes on them, the plant was around 90-100 cm tall.

It looks to belong to the family Cucurbitaceae Myriam but I can't find anything to match it. The fruit looks like a small watermelon!

Thanks Janet, I have also thought Cucurbitacea and also didn't find anything like it..

I got a bit closer by my half-Brazilian friend who says the popular name is 'Mata-Cavalo' which means 'Horse Killer' !
many different species seem to be named Mata-Cavolo though..
But for sure it in the family Solanaceae..
There is an enormous list of Solanum species recorded for the State of Rio de Janeiro!
http://floradobrasil.jbrj.gov....
I have done a big bunch of that but haven't found an exact match yet. I think I'll have to leave it as Solanum sp.

I looked at Solonaceae too! I'll take another look with 'horse killer' Mata-Cavalo.

Solanum palinacanthum maybe?

http://www.google.co.uk/search...

http://calphotos.berkeley.edu/...

That looks very close Janet! I'm still not sure though as the spikes look longer and more abundant on that one and the veins of the leaves more pronounced, another thing is that the ripe berry is yellow instead of red like I'm sure mine had.
http://calphotos.berkeley.edu/...

As one fruit is green, the other might not be fully ripe. It could be the angle showing the leaves differently, I can see a pronounced vein on your leaves, look at the one which is cut off on the right.

I think the strength of the spines might be to do with age, fruits are often on newer growth.

http://www.flickr.com/photos/2...

I tend to agree though that it doesn't quite match up, I found that by googling "'Mata-Cavalo Brasil" and only found the one photo for a start showing a flower and some spines so it was a good bet.

I have searched for Solanum fruits, I went to the 'show more results' and have found Solanum viarum. That is in the area and wide spread in Brazil. The description and most of the photos suggest the fruits turn yellow, but I found otherwise!

http://www.shirleydenton.com/p...

http://floradobrasil.jbrj.gov....

http://www.bugwood.org/weeds/f...

http://www.eattheweeds.com/ufo...

NOT EDIBLE: Solanum viarum, Solanum ciliatum, Solanum carolinense, all called Tropical Soda Apple or Horse Nettle, the first two turn red when ripe, the latter yellow. They usually start out with mottle green fruit. Some are toxic when green, others more toxic when ripe. Edibility is doubtful, botantical references vague, identification difficult. Best avoided. Of the three ripe S. ciliatum, now called Solanum capsicoides, might not kill you.

As Solanum ciliatum also has red fruits I checked that too, only found a drawing and that might not be correct!

http://www.printsoldandrare.co...

Another drawing which doesn't look like yours ..

http://www.oldimprints.com/Old...

I think Janet that you have found it! It really does look like Solanum viarum! The arrangement of the spines on the leaves look identical too, like on this picture
http://www.shirleydenton.com/p...
They say though the ripe fruits are yellow but like you found it described red on the next link I think we can safely go for Solanum viarum, S.ciliatum doesn't really match..
Thanks a lot!

S. ciliatum is now called Solanum capsicoides (as in the quote) but it still doesn't fit.

I think S. viarum is a match.