A few weeks ago Zimbabwe's fuel reserves fell to the dangerously low point of four days' supply, the lowest since independence. "You could say we were almost out," remarked one government official who blamed logistical problems for the drop.

South African Railways, the main transporter of Zimbabwe's imports and exports, responded to the near-crisis by making "an extra effort" to expedite the movement of gasoline tankers into Zimbabwe until reserves were built up again, even taking the unusual step of allowing some tankers to cross the border.

The incident, which went unmentioned in public by both governments, was a rare resort to the carrot for Pretoria. In recent months it has been using the stick quite a lot to drive home to the socialist government of Robert Mugabe that its economic well-being is closely tied to cooperation from Pretoria -- which, in turn, has a political price.

Haggling about that price has been the main reason for a deterioration in relations since Zimbabwe became the newest member of the Organization of African Unity 18 months ago and turned the Limpopo River into an ideological and geographical barrier between the two countries.

Pretoria's actions have left many here with the impression that it is deliberately trying to keep Zimbabwe from becoming a viable economic and ideological rival in this region. "I suspect there is a top-level feeling that they don't want what they think is a communist state to be a success," said one white Zimbabwean businessman who at the moment intends to stay in his country.

Others take a mildly more benign view. "I don't think South Africa has made a conscious effort" to undermine Zimbabwe and to "promote destabilization, but they are not about to bend over backward to help," said one Western diplomat.

His country, like most major Western powers, has pledged millions of dollars in aid to Zimbabwe. The aid is to help it become an economically successful, politically stable, multiracial society, in response to Pretoria's position that black rule means economic disintegration and political strife.

So far, South African moves have tended to undermine that Western effort rather than bolster it. Pretoria has: refused to renew a longstanding preferential trade agreement that gave Zimbabwean manufactured goods a guaranteed market in South Africa; refused to extend the lease on a Boeing jet for Air Zimbabwe; given notice that work permits of about 20,000 Zimbabweans working in South Africa would not be renewed when they expired, and begun requiring visas for Zimbabweans, a process that takes 14 days.

It is in railway matters, however, that Pretoria has attempted to wring political concessions out of Zimbabwe's economic needs. Last spring South African Railways withdrew 25 of its diesel locomotives in service with Zimbabwe's railways when their leases ran out, even though Zimbabwe desperately needed them to transport a record corn crop.

When Salisbury later let it be known that it would like to rent 45 diesel locomotives, the answer came back that Zimbabwe could have 60 if it liked, but that the request must come in a ministerial-level meeting. Wanting to stand by its decision against Cabinet contacts, and fearing that Pretoria would use such a meeting to embarrass them, the Zimbabweans refused and offered a written request signed by a top civil servant.

Pretoria declined, saying any written request must be signed by a minister.

The back-room bargaining on this is not over. Last month two senior Zimbabwean civil servants, one white, one black, slipped quietly into South Africa for secret talks with South African Railway officials in an attempt to break the impasse.

But Pretoria cannot bear the total blame for the worsening of relations. Zimbabwe has taken upon itself the tricky task of publicly condemning a government it regards as "immoral" -- and yet not biting so hard on the hand that feeds it that it ends up with only an elbow, or worse -- a cold shoulder from Pretoria. Given its dependence on South African rail and port facilities and the extensive business ties left over from the period before independence, that could be disastrous for Zimbabwe.

Like most other black African countries, Zimbabwe severed diplomatic relations with Pretoria, forced South African holdings in the local press to sell out and broke all sporting contacts. The South African diplomatic mission was downgraded to a trade office.

But there has also been a barrage of rhetoric against South Africa from Zimbabwe. The press seldom refers to it without the label "racist." And ministers repeatedly allege that Pretoria is destabilizing their country, although they offer no proof. There have been threats to nationalize South African companies and charges that Zimbabweans work in "slave labor" conditions in South African mines.

In private, Zimbabwean officials downplay this rhetoric. For example, asked how they can expect South Africa not to be angry when they publicly endorse economic sanctions against it (yet say they cannot honor them because of their economic dependence on Pretoria) Transport Minister Josiah Chinamano replied, "Then they must be angry with everyone in the world."

Supporting sanctions "is an international mode of condemning an iniquitous system in South Africa," said Deputy Economic Planning Minister Morton Malianga. "We are only joining the bandwagon."

But South Africa does not lightly dismiss this rhetoric, even if it can be ascribed to overenthusiasm of the new kid on the antiapartheid block.

Complaints about rhetoric apart, South Africa appears motivated by two other concerns in its relations with Zimbabwe. One is to ensure that it is not used as a staging base for guerrilla forces. The second is to impress upon Mugabe that without Pretoria's cooperation, his efforts to set up a regional economic union of black-ruled states as a counter to South African plans for a "constellation of states" are doomed.

On the first count, Salisbury "has the assurance from us that as long as it doesn't harbor terrorists, we won't lift a finger against them," one South African official said. His Zimbabwean counterparts feel they have sent an important signal to Pretoria by not allowing the African National Congress, South Africa's main black guerrilla organization, to establish official offices or bases here.

Presumably, events in Mozambique, which has given haven to the congress, are a foretaste of what Zimbabwe could expect if it follows suit. There, an antigovernment rebel movement, widely reported to have South African support, is fighting the forces of President Samora Machel over a widespread area.

If one agrees with Malianga about the basic reason for the tensions between Zimbabwe and South Africa, then they are likely to have strained relations for a long time to come. "It's race, pure and simple race," the Zimbabwean official said.